Solar Farm - Australia Neutral Connection 1

[acog]

Hi All,

I am looking at a large solar farm in Australia.

The current design is for several 33kV to 415V Dyn11 step up transformers with LV supply from approx 15 x 60kW 3 phase inverters per transformer (inverters will be mounted on or adjacent to large solar panels).

My question is- Do we run a neutral from the step up transformer to the inverters or not? I can't find anything definitive in the standards / codes here to rule out not running a neutral. The inverter spec is less than 5% THD.

I know most of you are not familiar with Australian codes so please assume IEC if in doubt.

At the moment we are looking at running the three phase conductors and an earth to each inverter.

Cheers

Andy

 

[Thumpstead]

What make and model is the inverter? Does the inverter have an isolation transformer?

 

[davidbeach]

I'd think that the transformer ought to be wye on the high side, not low side. Most inverters that meet the requirements of the North American market would have a delta(inverter)-wye(system) transformer, with the wye expected to be grounded. But not necessarily expected to carry any current, so no need for a neutral connection. But with your 33kV on the delta side, you will have overvoltage issues between the time the system clears a ground fault and the inverters go off line.

 

[acog]

Thumpstead: The inverter has not been selected yet but it will definitely not have an isolation transformer.

davidbeach: Agreed, the delta on the LV side would filter some harmonics as an added benifit as well. At this point in time the design is based around a Dyn transformer because this is the standard distribution transformer design in Australia.

I'm not sure why the overvoltages would be an issue with a Dyn transformer and not a YNd type?

I have seen 2 large solar farms in the north america and they both use a YNyn reticulation transformer, with a delta harmonic mitigaton transformer on the 33kV side back at the substation.

I thought this was odd.

 

[waross]

If the power is fed into the wye, there should not be a connection to the neutral, even if one is present. The wye point may be grounded if by so doing does not complete a neutral connection. The delta will stabilize the voltages and phase angles. Any phase angle error or voltage unbalance will lead to currents circulating in the delta, limited by the transformer impedance. Faults line to neutral on the wye side will lead to heavy circulating currents in the delta which will cause a backfeed into the fault. The two good phases will backfeed the fault through the delta winding. Better to leave the wye point floating.

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

 

[davidbeach]

Actually, having the backfeed into the fault is the whole point. Absolutely have to have an interrupting device on the that will detect the fault and remove the source.

The problem is that if there is a ground fault on the 33kV, for the period between when the opening of the substation breaker and the inverter going off line, the unfaulted phases will rise to phase-phase voltage phase-ground, aka a 73% overvoltage. Anything connected phase-neutral on the unfaulted phases will experience that overvoltage.

If the transformer in question were a wye-wye with both neutrals grounded, and an isolation transformer in the inverter cabinet, tripping the output of the inverter package will remove both the positive sequence source and the connection in the zero sequence network.

 

[FreddyNurk]

What does the utility state about the transformer configuration? I'd be surprised if the relevant utility wouldn't have certain requirements for a HV connection of this type.

 

[acog]

davidbeach: thanks for your comments. We will not be running a neutral conductor and will connect all small loads on the system in phase - phase configuration.

FreddyNurk: This will be a private installation. The utility requirements will not apply.

 

[Kiribanda]

acog,
The PV installation (solar fram) can be assumed as a cutormer owned generator.That means when here is an earth fault on the 33 kV utility side,there should not be overvoltages on utility side and still the network should run as an "effectively earthed" network for proper operation of utility earth fault protection scheme.This condition can only be satisfied by having a YNd11 transformer with earthed WYE on the 33 kV side.But if the transformer is having DELTA on 33 kV side then the generator will not sense any zero seq currents during an earth fault on 33 kV utility side and it will continue to opearte and feeding to the fault which is unsafe and not allowed.Therefore the tranaformer should be YNd11 with earthed WYE on 33 kV side and hence a neutral connection between Inverter and transformer is not required.Normally installing a YND transformer with earthed WYE on utility side is a prerequisite for connecting customer own generators(non utility or so called independent power producers IPPs)to a utility network.

 

[acog]

Hi Kiribanda,

If we used the configuration you are proposing, aren't we met with the same problem on the LV (delta) side where the PV array will continue to feed an LV reticulation phase to earth fault?

 

[waross]

Grounded wye on the utility side? delta on the other side?
This arrangement may result in a failed transformer.
Any voltage difference to ground between the utility phases will cause a circulating current in the delta limited by the transformer impedance.
This circulating current will try to increase the lower voltage phases.
Any phase angle error on the utility side will cause heavy circulating currents in the delta as the transformer bank tries to correct the phase angles.
If a phase is lost on the utility side, this transformer arrangement will try to "replace" the missing phase.
In the event of a phase to ground fault, the healthy phases will feed through the delta and contribute to the fault current.
If the distribution line has single phase voltage regulators installed, you may expect a lot of phase angle issues.
Solution: size the transformer or bank at 200%. Then when an unbalanced voltage takes out one of the wye side fuses the installation will continue to work in open delta configuration. (I believe this may be called a "V" connection in IEC land. A delta with one transformer missing.)
Have I missed anything David? grin

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

 

[Kiribanda]

acog,
If we used the configuration you are proposing, aren't we met with the same problem on the LV (delta) side where the PV array will continue to feed an LV reticulation phase to earth fault?
Sorry,I cannot clearly understand what you are trying to point out.Are you referring to the zero seq circulating current in the low side delta winding,during an earth fault on the 33 kV high side? Otherwise could you please use a simple sketch and expalin what is this circulating current?

 

[davidbeach]

Bill, all true, but...

High side requires CT based relaying with 3-phase interrupter. 33kV line has to be balanced well enough, and transformer robust enough. Certainly couldn't have any single-phase stuff (reclosers, regulators, etc.) between substation and point of connection.

Perhaps the Yg-Yg connection with an effectively grounded inverter would be better, but it is essential that the Solar Farm present itself at the 33kV level as an effectively grounded source.

Nobody ever said that a distribution circuit was designed for sources out on the circuit. Trying to make the circuit be something it was never designed to be can lead to compromises. Rather than subjecting all line-neutral connected customers on the remainder of the circuit to overvoltage conditions during faults, the pain all needs to be on the part of the party proposing the interconnection.

 

[waross]

To clarify, the solar farm will see the transformer as a delta wye. No problem.
The utility side will see the transformer as a grounded wye;delta. It is unbalances and faults on the utility side that will cause problems.
In a delta connection the delta must close. That means that the primary phases must all be equal in magnitude and at the proper phase angle. as an example, if one phase is 10% high on the utility side one leg of the delta side will be 10% too long to close the delta. But it is closed by a bolted connection. The 10% voltage difference will cause a current to circulate in the delta, limited by the impedance of the transformer.
Phase angle errors also cause issues.
On a transmission line with lots of protection and balanced loads you can live with this connection but it is a bad idea for distribution circuits.
If you remove one transformer from a delta, the other two transformers form a virtual transformer across the open side. In a bank with identical transformers this virtual transformer has the same properties as the transformer that was removed. Now if the primary voltage of this transformer is increased 10% the secondary voltage will also increase 10%. This has the same effect as connecting two transformers in parallel when the voltage differs by 10%. Sufficient current will flow to drop the voltage of the high transformer and raise the voltage of the low transformer until they match.
I have come across wye:delta banks where the load was about 20%. One transformer was cold, one was hot and the third transformer was smoking. Two transformers failed within a week or so.

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

 

[jghrist]

The concern for applying the grd-wye primary connection to a distribution circuit may be misplaced. The OP indicated that this is a private installation, which may mean that there is no load connected on the 33 kV system. The 33 kV system may be a collector system only and the connection to the utility may be at a 115-33 kV substation (or some other transmission voltage).

 

[acog]

jghrist is correct. The 33kV network will be a collector system only, which will be connected to a 33/132kV transformer. It would be reasonable to assume the 33kV network will be balanced at all times under normal operating conditions.