The differences between a Short Circuit Calculation and Load Flow Calculation are:
1. Motor MVAs are equal to the rated Equivalent MVAs.
2. Starting Motor MVAs are equal to 6 times rated Equivalent MVAs.
3. Static Loads are included in the calculations.
In this Load Flow Calculation, we assume that a 150 MVA motor is being started at the 400V Bus after all loads has been started.
In Load Flow, the upstream contribution is equal to the downstream. Available MVAs are equal to the ratio of the MVAsc of each branch.
At 11KV Bus:
Upstream: MVAt = 76.87 + 35.4 = 112.27 MVA
Downstream: MVAt = 7.49 + 5.0 = 12.49 MVA
Voltage dip at the 400V Bus during motor starting
Vd = 2.97 / (2.97 + 0.9) = 76.7 % or 306.8 V
The above calculation indicates that when all loads are already running, starting a 150MVA motor at the 400V Bus is not possible as the voltage dip is lower that the 85% which is the minimum starting voltage for a motor. It means that operational procedures should be in placed to have efficient plant operations.
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Quoted” 1. Motor MVAs are equal to the rated Equivalent MVAs.”
I wish to confirm that Motor MVAs is the short circuit MVA please? Thank you
Hi Tiong,
Motor MVAs in load flow calculation is the rated motor MVA, not the short circuit MVA
@ 11kv bus as per mva diagram,
upstream values mva are 67.33 (transfo. 2) and 44.94 (motor 5mva)
downstream values mva are 8.55 (transfo. 1) and 3.94 (generator side)..
Please help me to explain sir how did these values came. How to perform this calculation…Thank you.
Hi artem,
Here is how the values were taken.
Upstream Values:
The sum will be:
Downstream Values:
The sum will be:
Hope this helps.
exactly sir ver, this helps a lot. thank you very much..mabuhay ka and god bless.
I’m sorry sir Ver but i have additional query about the voltage dip or voltage drop upon starting the motor 150kva on the system which draws a power of 0.9 MVA..I have a result by hand calculations at 400V Bus: 77.22%, at 6.6kV: 87.72%, at 11kV: 89.28% which is a little different result on your MVA Diagram as shown that at 400V Bus: 76.7% (shown calculation), at 6.6kV Bus: 80.6%, at 11kV Bus: 90% and 33kV Bus: 96.6%.
Please help me sir, how did you perform this calculations.. thank you..
Hi artem,
I have updated the article. I hope it will be much clearer at this time.
Thanks
I salute you sir for sharing your knowledge and expertise, thank you very much..god bless.