Project background:

The client, who is a steel melting and manufacturing company in Illinois, USA, wanted to increase production output capacity by increasing the number of heats per shift from five (5) heats to seven (7) heats per shift. Each production shift is limited to 10 hours.

Electrical system:

The client’s existing electrical substation include two (2) main transformers labeled #1 Main and #2 Main transformers. Each transformer is rated as follows:

66MVA
140.9 KV NLTC +/-5% to 34.5 KV, Delta/WYE
ONAN/OFAF/OFAF, 39.6/52.8/66 MVA 65 Deg rise
7.23%Z

The above transformers are fed from the local utility servicing the area. Another transformer feeding the furnace unit is located inside the manufacturing building. That transformer is labeled as furnace transformer #8. The specification of the furnace transformer is as follows:

107MVA
34.5kV Delta primary
1200V to 593V Delta secondary (with 21 taps)
4.59%Z

Design approach:

In order to achieve two (2) more heats per shift, the existing heat duration must be shorten since each shift is limited to 10 hours and could not be extended. This means that the power output of the furnace transformer need to increase in order to achieve the same amount of work in shorter time.

To achieve this, transformers #1 Main and #2 Main will be connected parallel at the secondary terminals and then feed the furnace transformer. The two 66MVA transformers will have sufficient capacity even if furnace transformer rated 107MVA is loaded to full rating.

After running load flow and short circuit analysis, I found out that at 79.3MVA load rating of the furnace transformer, two (2) more heats per shift can be achieve. I recommended that #1 Main and #2 Main transformers be tapped at 133.86kV at the primary side while the furnace transformer must be tapped at 894V at the secondary side.