Calculate Size of Neutral Earthing Transformer (NET)
February 6, 2018 17 Comments
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
JIGNESH PARMAR
Dear sir, ATS (auto transfer switch) symbol fill in Single line diagram
Thanks&Regards K.Balaraman Ayyan electricals 9840768817
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Thank you , Sir Jignesh .regard isaac chibwe ampnet electrical services.
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!
Very good knowledge sir
Thank you sir for value able post
Kindly explain how you get the factors used in calculating the value of Xc
Please help HV to trive vector meter multification factor Formula
Dear Jignesh.Parmar
thanks for your explaniation
but what is the base for your equations which used in the calculation
thanks again
Sir, Here i found that the grounding transformer has secondary voltage. how can the grounding transformer have secondary winding. please reply
ok, Thank you. But how can we calculate the three phase four wire (star) AC impedance (resistance & reactance). Please give me answer.
Hello Jignesh sir how to calculate the connected load of passenger lift ( 8 persons). Please ,will wait for your reply.
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)
thanks. The primary winding of the earthing transformer is 11kV (line-to-line voltage) and not 11/sqrt(3) because during the ground fault, the voltage in non fault winding will increase from 11/sqrt(3) to about 11kV, correct? And where I can find references of the neutral Earthing Transformer overloading factor (of) ?.
Not a bad explanation. There are however many errors in the calculations which makes the article difficult to trust.
For example:
“Total Zero Sequence Capacitance (C)=0.006+0.0003+0.25+0=0.47730 µ Farad”
The actual maths is incorrect. It should be
0.006+0.0003+0.25+0=0.0.2563 µ Farad
Also the next line.
(Xc)=10×6 / (2×3.14x50x0.47730)=6672.35 Ω/Phase
The solution provided here is also incorrect.
Due to the number of errors I would not trust these calculations.
Good explanation, kindly inform which standard has been followed
Dear sir ,what is the size of cu strip to use for 2000kva or2500kva size transformer or dg nutral earthing? And what is short circuit with stand time for cu and c factor of copper?