Transformer loading

[Shahzad]

Hi everyone,

I have a 3MVA transformer which was installed when the plant was designed 15 years ago. This transformer is rated for 3MVA, 400V, 3 phase, 50 Hz. Transformer secondary incomer breaker is 5000 Amps with 5000Amp, 65kA continuous bus. As per design documents, transformer max load mentioned was 2.7MVA. However, site has collected data and they said maximum load (current they have seen) is 2200Amps, meaning 1.6MVA when plant is operating at full capacity. I have to add an additional load of 1MVA to this transformer. Is there any safety margin I should keep in mind while adding new load? e.g. A big new load would be a 500kA pure resistor, 3 phase, 400V heater. I am just curious how can I avoid spurious tripping if all of the load starts at the same time specially 500kW heater.

(2) Currently, there is no space beside existing LV switchboard (5000A), so I need to procure and install a stand alone switchboard to feed new load and feed it from the same 5000A switch board bus via cables. Should I consider choosing cables at 5000A*1.25=6250A. So, should I run 1000 MCM (10 cables per phase) in parallel to feed the new LV switch board? Any recommendation how this can be achieved?

Thanks in advance for the comments.

 

[augie47]

For that size current have you considered feeder buss ?

 

[Shahzad]

For that size current have you considered feeder buss ?

Can you explain little bit more about feeder bus? so instead of cable if I were to use feeder bus, how it would be better in terms of design? Space wise there are cable trays all over the place inside MCC, so not sure how feeder bus will run between existing LV switchboard and new one?

 

[augie47]

Simply an alternative wiring method. Might be worth considering cost-wise.
"Cost" is not an issue in my job and I mention it simply because I see ir fairly often on high amperage runs.

 

[masterinbama]

Can you get the switchboard manufacturer to make a new section that will bolt to the existing?

Sent from my moto e using Tapatalk

 

[JEFF MILLAR]

Feeder bus is great, but the application should consider the length. Long runs should limit the application. And straight runs are ideal.

 

[JEFF MILLAR]

Feeder bus is best for short runs. What is the nameplate impedance of your transformer. Would appreciate more information and would like to follow your as - built. You have cable tray all over. That may be your best raceway.