Questions regarding transformer sizing

[J. Hofstetter]

I work in the Mining Industry and the company that I worked for was in the design phase of building a new secondary stone plant. We used AggFlow software to help determine motor HP. I determined what size of pad mount transformer I needed to run the equipment and we bought a used transformer for this project. Since then, the company sold out to a competitor and they have decided to add even more equipment. I now have the issue of not having enough transformer to do the job. Due to the cost of buying another transformer, the new company wanted me to look into the option of having our local utility supply me with secondary power. So this is where my question comes from.
The way that I have determined what size of transformer that I need for these types of projects is to use the FLA from the NEC for each motor minus the largest motor FLA and find out what my 3 phase KVA would be. Then I would add 3 times the largest motor HP. I understand that if I used some type of a controlled start, whether it is a soft start or vfd on the largest motor I may not need 3 times, but this usually adds a little extra to the system and helps prevent voltage drop on startup. In this case the largest motor is 300 HP and I will be using a current ramp soft start. I have seen times where people use the exact same idea, but they do not find 3 phase KVA. This always made me a little weary, but if the system doesn't start loaded then they are probably alright. My local utility only wants to supply me with 1KVA per HP and as a matter of fact that number produced an odd size transformer so they want to actually step down a little from that since the motors don't run fully loaded. My calculation came up with 2400 KVA for startup so I say that I need a 2500 KVA transformer and the local utility wants to provide me with 1500 KVA for 1609 HP. Who is right? Am I way over or are they way under?
Any insight would be greatly appreciated.

thanks,

 

[Jraef]

I work in the Mining Industry and the company that I worked for was in the design phase of building a new secondary stone plant. We used AggFlow software to help determine motor HP. I determined what size of pad mount transformer I needed to run the equipment and we bought a used transformer for this project. Since then, the company sold out to a competitor and they have decided to add even more equipment. I now have the issue of not having enough transformer to do the job. Due to the cost of buying another transformer, the new company wanted me to look into the option of having our local utility supply me with secondary power. So this is where my question comes from.
The way that I have determined what size of transformer that I need for these types of projects is to use the FLA from the NEC for each motor minus the largest motor FLA and find out what my 3 phase KVA would be. Then I would add 3 times the largest motor HP. I understand that if I used some type of a controlled start, whether it is a soft start or vfd on the largest motor I may not need 3 times, but this usually adds a little extra to the system and helps prevent voltage drop on startup. In this case the largest motor is 300 HP and I will be using a current ramp soft start. I have seen times where people use the exact same idea, but they do not find 3 phase KVA. This always made me a little weary, but if the system doesn't start loaded then they are probably alright. My local utility only wants to supply me with 1KVA per HP and as a matter of fact that number produced an odd size transformer so they want to actually step down a little from that since the motors don't run fully loaded. My calculation came up with 2400 KVA for startup so I say that I need a 2500 KVA transformer and the local utility wants to provide me with 1500 KVA for 1609 HP. Who is right? Am I way over or are they way under?
Any insight would be greatly appreciated.

thanks,

You are right if you (as the end user) are going to own the transformer and do not want a severe voltage drop on start up.

But utilities are their own bosses, they can do what they want in terms of stressing the transformers, plus they have to contend with loading on the primary side conductors. So they will often pull something like this, where they will purposely overload the transformer, then tap it to make up for the voltage drop, but stick you with the problem of trying to start your motors. If the transformer fries, they have to replace it (but you will suffer the down time). It sucks, but I can attest to the fact that your protesting will be to no avail. They will do this and change blown fuses a dozen times before they get tired of hearing the user bitch about it and increase the transformer size, then bill them for the cost.

I had one crushing plant in Southern California where I warned them that the supply was under sized, they ignored me. One day about 5 years later the 110kV primary lines melted, fell to the ground and started a wildfire. THEN they decided that the transformer and feed was too small, but hit up the owner for the cost to increase the capacity, even though they had NEVER increased their load from when I did the original installation. It's just the way it is. My ex-partner is still doing this kind of work and he now almost always talks the owners into buying bulk power (110kV feed) from the utility and THEY own and maintain the transformers.

 

[J. Hofstetter]

Thanks for the feedback Jraef. With the old company we always purchased primary power which meant that we owned and serviced the transformers and this was never an issue. Your comment about changing the taps to help with the voltage drop was exactly what I thought and feared. With sensitive electronics (Plc's, VFD, and such) that higher than normal voltage tends to lead to problems. They may only be nuisance shutdowns, but they are shutdowns and they cost money.

 

[tish53]

We have 17 stone aggregrate quarries. We also have difficult time with utility sizing of transformers. We have developed relationships with most or our utility providers to get them to understand our needs better and "oversize" the transformers ( their point of view, our point of view is "right sizing") Sometimes we have to pay a excess facility charge to the utility, but we find it is worth the small monthly cost. We have found a 70% diversity factor works well in going from connected HP to running load. One caveat is our mech engs design our conveyors conservatively and we run very few motors up near full load.