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Improving Electrical Supply System for Lower Fault MVA: Analysis and Solutions
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[QUOTE="topcat123, post: 5465390, member: 465742"] [h2]Homework Statement [/h2] FIGURE 3 shows part of an electrical supply system. Draw the impedance diagrams reducing to a single impedance. Calculate the fault MVA levels in each unit for a fault on the left 3.3 kV bus with the bus section switch B open and A closed. Use a base MVA of 10 and show the fault MVA levels on a diagram. Comment on the result and suggest a method of improving the system to limit the fault MVA through T1 to approximately 5 × FL current. (It is not necessary to recalculate.) [h2]Homework Equations[/h2][h2]The Attempt at a Solution[/h2] The fault MVA and pu impedances [tex]G_1 and G_2\text{fault MVA} = \frac{16}{0.8}=20MVA[/tex] [tex]G_1 and G_2\text{ pu impedance} = \frac{10}{20}\frac{20}{100}=0.1pu[/tex] [tex]G_3\text{fault MVA} = \frac{1.6}{0.8}=2MVA[/tex] [tex]G_3\text{ pu impedance} = \frac{10}{2}\frac{20}{100}=1pu[/tex] [tex]T_1,T_2and T_3\text{ pu impedance} = \frac{10}{10}\frac{5}{100}=0.05pu[/tex] So we need to work out the total impedance and fault current. [tex] T_2||T_3=0.025pu[/tex] [tex]G_3+T_2||T_3=1.025pu[/tex] [tex](G_3+T_2||T_3)||G_1||G_2=0.0477pu[/tex] [tex][(G_3+T_2||T_3)||G_1||G_2]+T_1=0.0977pu[/tex] So the fault MVA [tex]\frac{10}{0.0977}=102.4MVA[/tex] So the fault MVA through T[SUB]1[/SUB] is 102.4 and through the rest [tex]\text{voltage MVA}(G_3+T_2||T_3)||G_1||G_2=102.4*0.0477=4.88MVA[/tex] Fault through G[SUB]1[/SUB] and G[SUB]2[/SUB] [tex]\frac{4.88}{0.1}=48.88MVA[/tex] Fault through G[SUB]3[/SUB]+T[SUB]2[/SUB]||T[SUB]3[/SUB] [tex]\frac{4.88}{1.025}=4.76[/tex] [tex]\text{voltage MVA G[SUB]3[/SUB]}=4.76*1=4.76MVA[/tex] voltage MVA over T[SUB]2[/SUB]||T[SUB]3[/SUB] [tex]=0.025*4.76=0.119MVA[/tex] Fault through T[SUB]2[/SUB] and T[SUB]3[/SUB] [tex]\frac{0.110}{0.05}=2.38MVA[/tex] I think this is correct. The part I am struggling with is "Comment on the result and suggest a method of improving the system to limit the fault MVA through T1 to approximately 5 × FL current. (It is not necessary to recalculate.)" Any help would be appreciated. Thanks [/QUOTE]
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Improving Electrical Supply System for Lower Fault MVA: Analysis and Solutions
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