**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**

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