Sizing a generator to test a 7 kva transformer

[Mmcgar]

Hi all, I'm just wondering if anyone can confirm what I'm doing is correct. I have a cabin that contains basic electrical items (lights, sockets, computer station etc) it gets its supply via a 7 kva transformer (440v primary / 230v secondary). We want to test the transformer is working correctly and will be hiring a generator to do this. I just want to make sure there's nothing special to sizing this generator as iv never done it before. My thinking is that allowing for 20% losses in the transformer I'd size the generator by doing 7000/0.8 = 9.2 kva, then to give it a round number I'd look for a 10 kva generator. Is this a suitable way of doing it or is there more to it?
Thanks in advance

 

[bernardgreen]

There is quite a lot more to it.

Is this a test before first use ? Or has it been ( is being ) used and there is reason to believe it may not be in good condition.

A full test functional would required insulation testing Primary to Secondary and both to core and frame. It would also probably require a flash test where a few thousand volts is applied for a few seconds or minutes and then another insulation resistance test ( after the high flash test voltage has been removed ).

 

[Mmcgar]

Hi, thanks for the reply
the the transformer itself has been factory tested by the manufacturer and has certification of conformity. We just want to hook it into the control cabin and make sure everything functions ok.

 

[bernardgreen]

The Certificate of Conformity suggests it is in full working order and has adequate insulation. ( Suggests is the correct word if the manufacturer is an over seas low cost supplier )

Why can't it be connected to the 440 volt supply and its output voltage verified as being that which is suitable for the cabin.

Measure it with no load and then with maximum load + 5%

You can use a few kettles ( total 8 kW ) to load the device for the full load voltage test. ( remember to fill them with water )

 

[JohnW2]

...it gets its supply via a 7 kva transformer (440v primary / 230v secondary). We want to test the transformer is working correctly and will be hiring a generator to do this.

I think I must be missing something. From where does/will this transformer normally get its supply and, depending upon the answer to that, why do you need a generator to test it?

Kind Regards, John

 

[ban-all-sheds]

I'm also missing something, which is an understanding of why someone doing electrical work in an environment where there is a 3-phase supply, and something called a "control cabin", has to ask questions about how to do his job on a DIY site.

 

[ericmark]

Our transformers were around 250kVA each we had 12 from memory and out generators were 2 x 500kVA and 8 x 350kVA we needed to get 5 generators on line before connecting the first transformer. Once the first was connected then second and third were no problem but that first one was a real problem as the non transformer load was only around 50kVA and trying to get 5 generators on line to share such a low load before the first transformer was connected was a real problem. So easy to knock one out on reverse current when manually lining them up using droop. OK the load would not have been removed from the step down transformer and so really we were powering up two transformers both step up to 3.3kV and step down but this was a real situation and shows the problems with in rush to a loaded transformer.

Even small transformers I remember on a construction job we had some 110 volt transformers 5kVA three phase which would happy start on a B32 but others would require a few tries with D32 MCB's it was just down to way it was wound. There is no one size fits all transformers do vary on the inrush and loop impedance even for the same size.

The generators and another variable we had 1.5kVA and 2.5kVA Lister engined generators both same make and same basic construction. If we overloaded the 1.5kVA it would burn out but if we overloaded the 2.5kVA the field would collapse with the latter running a shearing clippers was a problem we could not start motor but if we switched on the shaping stone first it would take a few attempts to reach speed but once running we could use the clippers. So some generators will take a short overload others will not and with in some cases a start load 10 times run be it transformer or motor that's a real problem.

Also there is reliability on one job we needed around 1kVA to run aircraft warning lights on the cranes but the generators hired in were forever breaking down so we used 10kVA generators as these did not fail as often. It seems the larger the generator the better constructed in general of course.

If it were my job I would one hire not buy at least to start with and go for one at least twice the size of transformer. One could use a resistor start but likely you would find it hard to buy one off the shelf. Likely it would need making to order.