5. Calculate Size of Neutral Earthing Transformer (NET)

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