Calculate Size of Neutral Earthing Transformer (NET)

Calculate Size of Neutral Earthing Transformer (NET) having following details

 Main Transformer Detail :

  • Primary Voltage(PVL): 33KV
  • Secondary Voltage (SVL): 11 KV
  • Frequency(f)=50Hz
  • Transformer Capacitance / Phase(c1)=0.006 µ Farad
  • Transformer Cable Capacitance / Phase(c2)= 0003 µ Farad
  • Surge Arrestor Capacitance / Phase(c3)=0.25 µ Farad
  • Other Capacitance / Phase(c4)=0 µ Farad

Required for Neutral Earthing Transformer:

  • Primary Voltage of the Grounding Transformer (Vp) =11KV
  • Secondary Voltage of the Grounding Transformer (Vs) =240V
  • Neutral Earthing Transformer % Reactance (X%)=40%
  • % of Force field condition for Neutral Earthing Transformer (ff) =30%
  • Neutral Earthing Transformer overloading factor(Of)=2.6
  • Neutral Earthing Transformer Base KV (Bv) =240V=0.240KV

Calculation:

  • Phase to Neutral Voltage (Vp1) =SVL /1.732
  • Phase to Neutral Voltage (Vp1) =11 /1.732 = 6.35 KV
  • Phase to Neutral Voltage under Force Field Condition (Vf) =Vp + (Vpxff)
  • Phase to Neutral Voltage under Force Field Condition (Vf)=6.35+ (6.35×30%) =8.26KV
  • Total Zero Sequence Capacitance (C)=C1+C2+C3+C4
  • Total Zero Sequence Capacitance (C)=0.006+0.0003+0.25+0=0.47730 µ Farad
  • Total Zero Sequence Capacitance Reactance to Ground (Xc)=10×6 / (2×3.14xfxC)
  • Total Zero Sequence Capacitance Reactance to Ground (Xc)=10×6 / (2×3.14x50x0.47730)=6672.35 Ω/Phase
  • Capacitive charging current/phase (Ic)=Vf / Xc
  • Capacitive charging current/phase (Ic)=8.26×1000 / 6672.35 = 1.24Amp
  • Total Capacitive charging current (It) =3xIc
  • Total Capacitive charging current (It) =3xIc =3×1.24 =3.71Amp
  • Rating of Neutral Earthing Transformer (Pr)=VpxIt
  • Rating of Neutral Earthing Transformer (Pr)=11×3.71=40.83KVA
  • Size of Neutral Earthing Transformer (P)=Pr/ Of
  • Size of Neutral Earthing Transformer (P)=40.83/ 2.6 = 16KVA
  • Residual Capacitive reactance (Xct) =Xc/3
  • l Capacitive reactance (Xct) = 6672.35 /3 =2224.12Ω
  • Turns Ratio of the Grounding Transformer (N)= Vp/Vs =11000/240 =45.83
  • Required Grounding Resistor value at Secondary side (Rsec)=Xct/NxN
  • Required Grounding Resistor value at Secondary side (Rsec)=2224.12 /45.83×45.83 =1.059 Ω
  • Required Grounding Resistor value at primary side (Rp)=Xct
  • Grounding Resistor value at primary side (Rp)= 2224.12Ω
  • Neutral Earthing Transformer Secondary Current=P/Vs
  • Neutral Earthing Transformer Secondary Current=16000/230= 65.44Amp
  • Neutral Earthing Transformer Secondary Resistor Current (for 30 Sec)=1.3xItxN
  • Neutral Earthing Transformer Secondary Resistor Current (for 30 Sec)=1.3×3.71×45.83=221.18Amp
  • Neutral Earthing Transformer Reactance Base(Base X)=BvxBv/P/1000
  • Neutral Earthing Transformer Reactance Base(Base X)=0.24×0.24/11/1000=3.67Ω
  • Neutral Earthing Transformer Reactance in PU (Xpu)=X% =40%=0.04Pu
  • Neutral Earthing Transformer Reactance (X)=Xpu x BaseX
  • Neutral Earthing Transformer Reactance (X)=0.04×3.67 =0.15Ω
  • Neutral Earthing Transformer X/R Ratio=X/Rsec
  • Neutral Earthing Transformer X/R Ratio=0.15/1.059 = 0.14
  • Fault current through Neutral (single line to ground fault) (If)=Vp1/Rp
  • Fault current through Neutral (single line to ground fault) (If)=6.35×1000/2224.12 =2.86Amp
  • Short time Rating of Neutral Earthing Transformer=PxOf =16×2.6 =41KVA

Result:

  • Rating of Neutral Earthing Transformer (P)=40.83/ 2.6 = 16KVA
  • Short time Rating of Neutral Earthing Transformer=41KVA
  • Ratio of Neutral Earthing Transformer =11000/240 Volt
  • Neutral Earthing Transformer Secondary Current=65.44A
  • Required Grounding Resistor value at primary side (Rp)=2224.12Ω
  • Required Resistance at secondary side (Rsec)= 1.059Ω
  • Neutral Earthing Transformer Secondary Resistor Current (for 30 Sec)= 221.2A


February 06, 2018 at 07:02PM by Department of EEE, ADBU: http://ift.tt/2AyIRVT
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