Backfeeding through a transformer question 1

[Waqalevu]

Due to the failure of our site standby generator, we have had to install a contingency backup supply on a critical area of our plant. The contingency arrangement comprises two 1250 kVA generators connected in parallel to a 400/690 Volt transformer. The transformer is 690 Volt secondary is connected to our MCC busbars. The hire cost for this equipment is huge, and it is likely that we will require this for many weeks.

There is an alternative, albeit we will be very limited with what we can run. I could connect one of the generators to the secondary of a 11kV/400V, 1250 kVA transformer. I can then run the generator if required, using this transformer as a step up and back feed onto the 11 kV bus. The MCC that requires the contingency power is split in two halves, with two incomers and a bus coupler. The bus coupler is normally open, and each half of the board is supplied from a 11kV/690V, 3000 kVA transformer. So what I'm thinking of doing is using the 11kV/400V, 1250 kVA as a step up, with the generator connected to the 400V secondary. The 11 kV would then supply one of the 3000 kVA transformers, stepping the voltage back down to the required 690 Volts. The other transformer would be switched off. I would then limit what I could run, up to the capacity of the 1250 kVA transformer.

This would save the cost of hire of one generator and the step up transformer and cables.

Appreciate if anyone can see any pitfalls with this arrangement.

 

[ScottyUK]

You should be careful if the generator and 11/0.4kV transformers both have earthed neutral points. You should have only one neutral-earth bond, which means you need to separate the neutral-earth bond at one location.

 

[squeeky]

If I have read the question correctly here I go. This is a standard installation in our factories. We will install a 1850kVA AC Deisel Generator (DG) at 400 or 525v. There is a breaker on the machine. This feeds a transformer. The 2000kVA transformer steps up to 11kV and onto a bus system via an MV breaker where up to 28 MV breakers sit. We trip all breakers on a black out to avoid powering all transformers with a small DG. The 11kV to 525v, 2000kVA transformer feeding the Main boiler remains in and so does the 11kV to 400v, 1000kVA Essential services transformer. Total MVA of transformers is now 5MVA. This enables us to have lighting, keep the control system functioning and to restart the first boiler. With the boiler running then we start a turbine connected to the Steam Turbo Alternator from which we get 2.5 to 4.0MVA to start the next boiler then the next turbine and so on.

Here is the catch. We are controlling the exciter from the 11kV side. As the voltage on the 11kV side changes we alter the excitation on the 525v exciter to maintain 11kV. If you are installing a retrofit LV AC generator where will you conduct your regulation?

If your existing breakers have reverse power then you will have to disable this.

You will have to watch the losses in the transformers. When you make your running phylosophy you must take these losses into consideration as you will not have the full capabilty of the AC Generator.

Next. Your machine needs to be able to identify when other machines join or leave the network. When you start up, the Diesel AC Genertor (DG) will have to be in voltage mode as it will only be able to regulate the voltage as the demand changes. When you close your bus coupler the controllers in the AVR need to idnetify this and move to droop mode. This allows the two generators to move up and down together. "Looking" through the transformer may require some tuning of the AVR. If you then bring in the Utility your DGs will have to move to Power Factor Mode. This is because you cannot influence the voltage and all you can do is import or export VARs.

One more warning. In the IEC 60076-1 in Clause 8.3 there is a requirement to specify your transformer as being connected to a generator when you order it. The withstand voltage is now 1.4 times for 5 seconds. This has to do with load rejection. We have not done this from ignorance on older plants and they have been running for 10s of years. Is it important. Yes, to someone, but we have survived so far.

If you need more please let me know and I can send some "Stuff."

 

[Marmite]

Another consideration is system grounding on the 11kV side. Depending on the transformer vector groups you might inadvertently be operating an unearthed 11kV system, which could overstress the insulation. The protection systems are unlikely to work as designed with the existing settings and configuration.
Regards
Marmite

 

[waross]

Have you considered an auto transformer? During shutdowns of the numerous heavy oil up-graders in this area generators are brought in to supply temporary lighting. Some of the plants have 600 Volt systems but 600 Volt generators are rare in this part of the world. 480 Volt generators are plentiful. Auto-transformers are used to boost the 480 V up to 600 V. In the field, auto transformers are used again to drop the 600V to 480 V for rental welders, and smoke extractors.
One option is to use boost rated 230:230 Volt in an auto-transformer wye connection. That would give you 800 Volts. Drop it 5% with the taps if possible to 760 Volts. Then dial the AVR down 10% until you get 690 Volts.


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

 

[prc]

waqalevu, it will be good if you indicate the vector numbers of the tarnsformer connectionos,like YNyn0 (star-star -,neutrals groynded on both sides)YNd11(HV star neutral earthed/delta with leading by 30 degrees) etc,

squeeky, you are referring to clause 5.3 of IEC 60073-3 ed3.0-2011. 1.4 times for 5 sec is not from voltage withstad angle,but from overfluxing (v/f) issue. So rather than V increase, you have to watch v/f increase. Normally all transformers can withstand such overfluxing ,provided rated flux density is 1.7 T or less.Such overfluxing can be a concern with large turbo generators, but may not be a concern with such small diesel generators. I am not sure. Such overfluxing problems in generator transformers came to light first time during early 1960's when 500-600 MW sets were commissioned first time in US and standards added such clauses for generator transformers.

waross, can you explain a little more how 800 V is obtained by auto connection?

 

[waross]

As I understand the generator voltages, the line to neutral voltage is 230 Volts. If a transformer is available, a 2:1 auto transformer will boost this to 460 Volts, line to neutral. 460 volts line to neutral will develop 800 Volts line to line.
Another option is to use two single phase 400 Volt to 400 Volt transformers in an open delta auto-transformer connection. We see both the wye connection and the open delta auto-transformer connections on the temp power sets.
This solution is contingent on suitable transformers being available. I am not familiar with the transformer voltages available in IEC land.

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

 

[Waqalevu]

Will get the info when next at site.

 

[squeeky]

Dear PRC. IEC 60076-1:2008 in teh section entitiled 8. Miscellaneous requirements

8.3 Load rejection on generator transformers
Transformers intended to be connected directly to generators in such a way that they may be subjected to load rejection conditions shall be able to withstand 1,4 times rated voltage for 5 s at the transformer terminals to which the generator is to be connected.

I am not at work and will look at teh reference that you gave me and try to learn this aspect. What you are saying is that it is possible for the v/f ratio to go out to a point where the transformer over fluxes. The voltage would rise and the frequency fall as it came into a fault condition. Hummm. Stupid me will have to ponder. Thanks for this. Or may be curse you as I will now need to find it digest it and learn this new info. Thanks anyway. Great to be in the discussion.

 

[prc]

squeeky, Dont worry.Every day we are learninmg. I made a mistake the Standard Nois 60076-1 and not 3. 2008 edition was updated in 2011.Also see clause 5.4.3 in this latest edition.