Voltage Drop Calculation with MVA Method

Voltage Drop (VD) is useful to determine cable sizing of electric power system. By selecting the appropriate cable  will  improve stability. To calculate voltage drop at bus we can use MVA Method. In this session we will use the same one line diagram on article before.

Look at the one line diagram below.

In this case study, we want to determine how is the voltage drop at bus 12 kV?. On article before, we got that MVAsc series is 153 MVAsc and MVAsc motor is 75 MVAsc. So, to calculate we use formula :

• VD at bus 12 kV = MVAsc series/(MVAsc motor + MVAsc series) = 153/(75+153) = 67%
• It means that the voltage at bus 12 kV decrease 33% when motor starting to run.
  

Now, we try to calculate VD at bus 69 kV.

• MVAsc for motor, transformer and line are connected series to the bus 69 kV. So, we have to sum up like a parallel network.
• 1/MVAsc series = 1/MVAsc motor + 1/MVAsc transformer + 1/MVAsc line = 1/(75)+1/(198)+1/(1230) = 52 MVAsc
• VD at bus 69 kV = MVAsc generator/(MVAsc generator + MVAsc series) = 1500/(1500+52) = 96%
• It means that the voltage at bus 69 kV decrease 4% when motor starting to run.

By using MVA method, we can solve the voltage drop esier than other method. To check whether the result is correct, you can use ETAP or similiar software.

Thank you.
Please comment after reading my posting.

Regards,
HI