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

About Jignesh.Parmar
Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical). He is member of Institution of Engineers (MIE) and CEng,India. Membership No:M-1473586.He has more than 16 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He has published numbers of Technical Articles in “Electrical Mirror”, “Electrical India”, “Lighting India”,”Smart Energy”, “Industrial Electrix”(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.

13 Responses to Calculate Size of Neutral Earthing Transformer (NET)

  1. bala raman says:

    Dear sir,             ATS   (auto transfer switch) symbol fill  in Single line diagram

    Thanks&Regards K.Balaraman Ayyan electricals 9840768817

  2. isaac chibwe says:

    Thank you , Sir Jignesh .regard isaac chibwe ampnet electrical services.

  3. Dennis says:

    This could be a great guide for electrical engineers looking to calculate the size of a neutral earthing transformer. It’s awesome that you listed the specific calculations for the voltages and frequencies for the main transformer. This information could be helpful for technicians who are running data centers, and looking for the exact voltage to get the job done. Thank you for such precise breakdown of the numbers!

  4. Shiv kumar says:

    Very good knowledge sir

  5. Vivekanand Pandey says:

    Thank you sir for value able post

  6. Anthony Dickson Ojih says:

    Kindly explain how you get the factors used in calculating the value of Xc

  7. N.dhetchinamoorthy says:

    Please help HV to trive vector meter multification factor Formula

  8. Dear Jignesh.Parmar
    thanks for your explaniation
    but what is the base for your equations which used in the calculation
    thanks again

  9. Mohammed Rafi says:

    Sir, Here i found that the grounding transformer has secondary voltage. how can the grounding transformer have secondary winding. please reply

  10. Md. Mostaq says:

    ok, Thank you. But how can we calculate the three phase four wire (star) AC impedance (resistance & reactance). Please give me answer.

  11. Pankaj Pal says:

    Hello Jignesh sir how to calculate the connected load of passenger lift ( 8 persons). Please ,will wait for your reply.

  12. Rafael says:

    Hello Mr. Jignesh: Why is the earthing transformer primary voltage (11 kV) a third of the three phase main transformer primary voltage (33 kV)? I have seem that many other authors say that it has to be the line to ground voltage of the main transformer (33 kV/1,73)

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