Captive Transformer

I will agree with RAMConsult about what he said, but will add that the reason for the dedicated transformer is to provide some isolation for transient protection but mainly to add impeadance to reduce inrush which will in effect produce a slightly softer start than across the line starting. I have had experience with motors around 30000 HP using isolation transformers but nothing as small as what you propose. It sounds like you have a very weak system. When I dealt with weak systems, I noted that the large motors always started better when everything else was running, particularly large inertia rotary loads. The other motors acted as generators during the voltage sag. This has nothing to due with the question but you need to analyze the loads connected and running when the motor is started. You also need to get full information about the power coming into the facility and do a complete system study. I have another question, is the 12.5 MVA the base rating and does that transformer have Fan (FA) or more rating? You have not mentioned what the transformer impeadances are.

4Wsilver

Thanks for your comments

Very interesting for me your information about your experience regarding large motors with large inertial constribution to reduce the effect of the voltage sag.

The 12.5 transformer are ONAN type, off-load tap changer (old version) 11% SC impedance.

Bye

Hi RAMConsult

It result very pleasant for me getting your comments.

The Captive transformer issue is another industry which are located far from the one of my former Post. In this case there are an strong system and the disturbance are such common for every grid in the world. Also no on-site generation exist because there are a 600 MW utility 5 KM far from the plant.

My doubts regading consultans recommendation was the cost-effective of captive transformer invesment and that after the 2500 KW motor installation the load is dangerously close to the transformer top capacities.

Thank a lot

Bye

As RAMConsult said, here is a lack of lot of data:

System short-circuit power. I'll take it 1000 MVA

Transformer short-circuit impedance. I'll take it 11%

Motor p.f. and efficiency. I'll take these 0.85 and 90%

Motor p.f. at start. I'll take it 0.3

Motor Istart/Irat. I'll take it 6

Load p.f. I'll take it 0.85

In this case the steady state transformer overload will be only 30 A.

However, at start-when transformer is 8 MW loaded-and the supply voltage will be 6.3 kV[by reducing 115 kV to 6.3 kV at the source] and total current at transformer terminals is 2600 A[2.26 times the rating] the voltage at transformer terminal does not drop to less than 6.124 kV[even more than 6 kV rated.]

If the motor is supplied from the transformer with 3*120 sqr.mm 10/6 kV copper conductor 500 m length the voltage at motor terminal at start will be 5860 V[97.6% from motor rated voltage].

I am curious as to why the process requirements would require DOL starting versus other methods.

The reasons for not allowed soft start switches is because in continuous process is a common practice to have available one main pump in operation and other in reserve (spare) so in case of the main pump failure, the reserve shall start as fast as possible to maintain the process parameter in term of flow, pressure etc and so avoid plant trip

Bye

That rationale makes precious little sense.