Back feeding step down transformer

[Toolnose]

Hi,

Looking for info on possible issues with back feeding a step down transformer. Currently have a mobile genset connected to the LV side of a 11kV to 415V delta star 2.5MW step down transformer. Normally this is used to supply power LV bus in one of our MCC's and kW limited to avoid back feeding. The breaker on LV incomer to the MCC has however failed and management have decided that we can back feed the transformer to prop up power supply when required. Site is island only with all other generators being 11kV. The LV genset is normally used as a back up when we bring one of the larger sets offline for servicing etc. I am not sure that I like the idea but have no solid argument to use against it so hoping to get an idea of potential issues.

Regards.

 

[cuky2000]

Transformers is a reversible device that can work in feeding it in any directions.
The protection system and relay setting should be careful evaluated to make sure work in a reverse mode.

 

[waross]

Is the wye neutral point solidly connected to the generator neutral?
A four wire wye/delta connection abhors any unbalance, either voltage or phase angle.
There are issues when a grounded wye/delta transformer is used on a distribution circuit.
For a close coupled connection from a generator to a transformer there should be no problem.
With a floating wye point there will be less issues.

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

 

[Toolnose]

Wasn't there when genset was originally connected but pretty sure genset neutral is connected straight onto neutral point on the tranny, so yes solid connection.

 

[FreddyNurk]

waross' point is pertinent, and to expand on cuky2000's comment, it is imperative that whatever you put in is capable of detecting an earth fault on the 11kV side, and disconnecting all sources of power.
As a Dyn transformer, there's no earth reference for when its the only source, there no doubt would be other earth references on the 11kV bus, but its guaranteed that they won't trip off your LV generator when the associated protection system detects a fault.

Some solutions might be an intertrip arrangement that drops the generator when the 11kV earth protection trips, or using a YNd transformer instead.
Of course, based on your description, you may already have this issue regardless of power flow.
Conceptually, this is also the same as the restrictions on solar generation and resultant requirement of anti-islanding for grid connected solar in Australia.

EDMS Australia

 

[Toolnose]

Should clarify, when I said back up the genset is not used stand-alone it will be synchronised with our other sets and provides extra power to give us some extra spinning reserve or shut down one of the bigger sets.

 

[RRaghunath]

The transformer being step-down type, the LV winding no-load voltage rating would be 433V or so. That being the case, the generator connected to 415V bus needs to develop 433V or so for it to be connected/synchronised to 11kV bus. If the load happens to be at 11kV predominantly, the VAR burden on this LV generator will be great as it has to overcome the voltage drop in the 415V/11kV transformer as well before it can feed the loads at 11kV.
Is 11kV/415V transformer with OLTC?
Is there a capacitor bank for VAR compensation at 11kV??

 

[Toolnose]

No OLTC or capacitor bank.

 

[FreddyNurk]

RRaghunath, that's not really a problem in practice, normally the generator would be wound up to 430V or thereabouts or possibly the tap on the transformer might be set to compensate. Ideally some sort of coordinated var sharing would be good too, but its possible to do with a cross-current scheme as well.

Toolnose, what you have is certainly possible, noting the floating of the low side neutral on the transformer.
If you already have a scheme for disconnection of all the sets on earth fault on the 11kV bus, including the LV set, not such a problem. If your system will allow you to feed the 11kV bus via the transformer's delta connection without a means to detect an earth fault on the bus, then it is an issue.
I know of at least one site that runs a generator and transformer to feed a 6.6kV bus using a Dyn transformer, but there are separate earthing transformers for the bus and the system is configured to trip all the generators on a fault. That transformer is a unit transformer though, and there's also differential protection on the generator-transformer combination, although it doesn't do anything for high side earth faults.

EDMS Australia