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2. Calculate Size of Capacitor Bank / Annual Saving & Payback Period

CALCULATE SIZE OF CAPACITOR BANK / ANNUAL SAVING & PAYBACK PERIOD

  • Calculate Size of Capacitor Bank Annual Saving in Bills and Payback Period for Capacitor Bank. * Electrical Load of (1) 2 No’s of 18.5KW,415V motor ,90% efficiency,0.82 Power Factor ,(2) 2 No’s of 7.5KW,415V motor ,90% efficiency,0.82 Power Factor,(3) 10KW ,415V Lighting Load. The Targeted Power Factor for System is 0.98. * Electrical Load is connected 24 Hours, Electricity Charge is 100Rs/KVA and 10Rs/KW. * Calculate size of Discharge Resistor for discharging of capacitor Bank. Discharge rate of Capacitor is 50v in less than 1 minute. * Also Calculate reduction in KVAR rating of Capacitor if Capacitor Bank is operated at frequency of 40Hz instead of 50Hz and If Operating Voltage 400V instead of 415V. * Capacitor is connected in star Connection, Capacitor voltage 415V, Capacitor Cost is 60Rs/Kvar. Annual Deprecation Cost of Capacitor is 12%.

 CALCULATION:

  • For Connection (1): * Total Load KW for Connection(1) =Kw / Efficiency=(18.5×2) / 90%=41.1KW * Total Load KVA (old) for Connection(1)= KW /Old Power Factor= 41.1 /0.82=50.1 KVA * Total Load KVA (new) for Connection(1)= KW /New Power Factor= 41.1 /0.98= 41.9KVA * Total Load KVAR= KWX([(√1-(old p.f)2) / old p.f]- [(√1-(New p.f)2) / New p.f]) * Total Load KVAR1=41.1x([(√1-(0.82)2) / 0.82]- [(√1-(0.98)2) / 0.98]) * Total Load KVAR1=20.35 KVAR * OR * tanǾ1=Arcos(0.82)=0.69 * tanǾ2=Arcos(0.98)=0.20 * Total Load KVAR1= KWX (tanǾ1- tanǾ2) =41.1(0.69-0.20)=20.35KVAR * For Connection (2): * Total Load KW for Connection(2) =Kw / Efficiency=(7.5×2) / 90%=16.66KW * Total Load KVA (old) for Connection(1)= KW /Old Power Factor= 16.66 /0.83=20.08 KVA * Total Load KVA (new) for Connection(1)= KW /New Power Factor= 16.66 /0.98= 17.01KVA * Total Load KVAR2= KWX([(√1-(old p.f)2) / old p.f]- [(√1-(New p.f)2) / New p.f]) * Total Load KVAR2=20.35x([(√1-(0.83)2) / 0.83]- [(√1-(0.98)2) / 0.98]) * Total Load KVAR2=7.82 KVAR * For Connection (3): * Total Load KW for Connection(3) =Kw =10KW * Total Load KVA (old) for Connection(1)= KW /Old Power Factor= 10/0.85=11.76 KVA * Total Load KVA (new) for Connection(1)= KW /New Power Factor= 10 /0.98= 10.20KVA * Total Load KVAR3= KWX([(√1-(old p.f)2) / old p.f]- [(√1-(New p.f)2) / New p.f]) * Total Load KVAR3=20.35x([(√1-(0.85)2) / 0.85]- [(√1-(0.98)2) / 0.98]) * Total Load KVAR1=4.17 KVAR * Total KVAR=KVAR1+ KVAR2+KVAR3 * Total KVAR=20.35+7.82+4.17 * Total KVAR=32 Kvar

 SIZE OF CAPACITOR BANK:

  • Site of Capacitor Bank=32 Kvar. * Leading KVAR supplied by each Phase= Kvar/No of Phase * Leading KVAR supplied by each Phase =32/3=10.8Kvar/Phase * Capacitor Charging Current (Ic)= (Kvar/Phase x1000)/Volt * Capacitor Charging Current (Ic)= (10.8×1000)/(415/√3) * Capacitor Charging Current (Ic)=44.9Amp * Capacitance of Capacitor = Capacitor Charging Current (Ic)/ Xc * Xc=2 x 3.14 x f x v=2×3.14x50x(415/√3)=75362 * Capacitance of Capacitor=44.9/75362= 5.96µF * Required 3 No’s of 10.8 Kvar Capacitors and

  • TOTAL SIZE OF CAPACITOR BANK IS 32KVAR

 PROTECTION OF CAPACITOR BANK

 SIZE OF HRC FUSE FOR CAPACITOR BANK PROTECTION:

  • Size of the fuse =165% to 200% of Capacitor Charging current.

  • Size of the fuse=2×44.9Amp * Size of the fuse=90Amp

 SIZE OF CIRCUIT BREAKER FOR CAPACITOR PROTECTION:

  • Size of the Circuit Breaker =135% to 150% of Capacitor Charging current.

  • Size of the Circuit Breaker=1.5×44.9Amp * Size of the Circuit Breaker=67Amp * Thermal relay setting between 1.3 and 1.5of Capacitor Charging current. * Thermal relay setting of C.B=1.5×44.9 Amp * Thermal relay setting of C.B=67 Amp * Magnetic relay setting between 5 and 10 of Capacitor Charging current. * Magnetic relay setting of C.B=10×44.9Amp * Magnetic relay setting of C.B=449Amp

 SIZING OF CABLES FOR CAPACITOR CONNECTION:

  • Capacitors can withstand a permanent over current of 30% +tolerance of 10% on capacitor Current. * Cables size for Capacitor Connection= 1.3 x1.1 x nominal capacitor Current * Cables size for Capacitor Connection = 1.43 x nominal capacitor Current * Cables size for Capacitor Connection=1.43×44.9Amp * Cables size for Capacitor Connection=64 Amp

MAXIMUM SIZE OF DISCHARGE RESISTOR FOR CAPACITOR:

  • Capacitors will be discharge by discharging resistors. * After the capacitor is disconnected from the source of supply, discharge resistors are required for discharging each unit within 3 min to 75 V or less from initial nominal peak voltage (according IEC-standard 60831). * Discharge resistors have to be connected directly to the capacitors. There shall be no switch, fuse cut-out or any other isolating device between the capacitor unit and the discharge resistors. * Max. Discharge resistance Value (Star Connection) = Ct / Cn x Log (Un x√2/ Dv). * Max. Discharge resistance Value (Delta Connection)= Ct / 1/3xCn x Log (Un x√2/ Dv) * Where Ct =Capacitor Discharge Time (sec) * Cn=Capacit…