Two Incoming Feeders connected to secondary of Two Winding Transformer. 1

[mcm00048]

Dear All,

I know that this question is a beginner's question, but it never came to me, that's why I turn to you.

I would like to know if it is possible to connect two feeders to a secondary of a transformer. Both provide different current, but at the same voltage. I give you a part of the scheme. I understand that what would be done is to connect these two feeders to a common LV Switchgear , and from there depart with a single line to the transformer. Attached is a descriptive Picture of my question.

Thank you very much, any help is more than welcome.

 

[waross]

Well yes, but:
How do you propose to control the KVA sharing ratio?
The KVA sharing may be adjusted by adjusting the voltages.
The sum of the cable KVAs may be greater than the KVA demanded by the load.
The KVA sharing ratio may be quite different than the kW sharing ratio.

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

 

[mgtrp]

I'm not sure of the jurisdiction that you're operating in, but there are often code restrictions on connecting different sized cables in parallel (assuming that's what's being done here—I'm not 100% sure from the description and SLD).

Cheers,
mgtrp

 

[jfpe]

What is the application? Is this a step up transformer with power limited 480V sources?

-JFPE

 

[prc]

Why there should be any problem in connecting loads in parallel to a transformer secondary? Two feeders are such a case only. In fact the great breakthrough in 1885 when inventing transformers was two- a closed magnetic circuit and connecting loads and transformers in parallel. Till then for several years, engineers were struggling with 'converters' -forerunners of transformers - having open cores( no-load current came several % of full load rating) and light bulbs in series to transformer secondary.

 

[waross]

Ah. Light bulbs in series. That takes me back.
I was quite young so it was a long time ago that I worked on series lighting circuits.
These were street lighting circuits, not Christmas tree lights.
Open circuit voltage cold hit several thousand Volts.

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

 

[mcm00048]

Dear all,

@jfpe The Project is a PV Plant. The transformer is 0,480 / 11 kV, and two incomings from two separate Inverters are connected, each one wich different power and current as per the Picture attached.
@mgtrp: Cables are sized equal, but using differnet amount of runs.

Feeder 1: 2x(4x240) mm2 Cu XLPE/SWA/PVC
Feeder 2: 3x(4x240) mm2 Cu XLPE/SWA/PVC

Then, the prmary side of 11kV transformer is connected to MV Swictchgear incoming feeder.
Could you be kind to provide examples of paralell conenction of several feeders in paralell in a single winding of Two winding Transformer?

Thank you very much in advance,

Miguel.

 

[prc]

mcm, it will not work. Normally in PV plants they are using three winding transformers, with two windings as primary and one secondary as HV. In hydro and combined cycle plants also such 3 winding transformers are used with two generators feeding to one transformer. The inter winding impedances will be selected to facilitate the nominal loadings in each winding.

 

[mcm00048]

prc, thank you for your answer.In case a LV Panel to Group both Feeders into one to feed the LV Side of transformer, shall the following conditions be fulfilled?
- Voltage Level = 480V Feeders
- Phase: Both Feeders shall be equally phased in order not provide lagging or leading voltages
- Frecuency, shall be both for both feeders, is there any tolerance?

Thanks

 

[rcw retired EE]

From the voltage rating shown on the one line,(11kV / 480V/480V) you have a three winding transformer: one 11kV HV winding and two 480V windings. Teh symbol shows just one 480V winding?

Usual design for the two 480V windings is to have a 480V breaker on each winding connecting to one of the feeders. E

If you parallel the 480V windings, as you imply, your short circuit levels will be much higher. You will also have to do some bus work to connect the eight 480V terminals in parallel (3 phase + neutral) x 2.

Look at the nameplate of the transformer. Does it show two or three windings and 3 or 6 480V phase connections?

 

[prc]

mcm, in solar projects these are never paralleled. I don't know the reason. May be something to do with power electronics of inverters?

 

[mcm00048]

Dear all, thank you for your answers, please find attached a Handmade SLD for my better Explanation. The System Looks like the attachment.I have placed a LV Panel before entering the LV Side of Transformer, would be necessary?

rcwilson, I do not have Name plate of transformer available. The drawing it was not correct. It is Two Winding 11/0,48 kV Transformer.
prc: Could you tell me where I could ask about this issue?

Here you can find the SLD.

Link


Thanks

 

[prc]

mcm, Please have a look at IEEE Std C57.159 that covers such Solar transformers. The type of connection that you are planning is not covered in that standard. I am checking with some experts for the feasibility of proposed connection.In case I get any response,I shall post.

 

[mcm00048]

Thank you prc, I will also have a look on other Standards.

 

[jfpe]

Have a conversation with the inverter manufacturer. Depending on the inverter topology, two separate LV windings may or may not be required. In general, if you are designing around 2 central inverters (564kVA and 376kVA) you will need two LV windings. If each input is a number of smaller 3-phase string inverters, you can use a single LV winding.

-JFPE

 

[krisys]

mcm00048 (Electrical)(OP)

Your system connection arrangement has the inherent tendency to cause circulating current. You cannot connect two sources to the transformer like you have shown in the sketch.
The branching like this is fine for the load side, but not on the source side. That too, the kind of sources, where the generation cannot remain constant.