Earthing Design Calculations for 5600 KVA Transformer

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anasshad

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Hello all. This is my first post in these forums so please ignore any mistakes I make.
Okay, so I'm a fresh Electrical Engineering graduate who has started working as a design engineer. I've been asked to design a earthing system for a 5600 KVA transformer (neutral and equipment earthing) but I have no idea how to start. Could any one please guide me through the process like a mentor?
Or just give me some clues about where to start. I have to provide IEC and NEC standards for each step.
Thank You.:)
 
Julius Right

Julius Right

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Electrical Engineer Power Station Physical Design Retired
At first, welcome at our forum. Second, any forum it could be only an idea source, but seldom a solution giver.
I agree with petersonra : it is not enough information.
However 5600 kVA it is not from IEC world.[recommended will be 5000 or 6300.]
Let’s say, the rated voltage could be 66/13.8 kV Delta-YN. The secondary neutral could be grounded.
According to IEC 60076-5 Table 1 – Recognized minimum values of short-circuit impedance for transformers with two separate windings the minimum short-circuit impedance is 7%.
The three phase solid short-circuit current will be I”k=5600/sqrt(3)/13.8/.07=3.35 kA.
However, according to IEEE Std 142-2007 IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems ch.2.5 Unit substations, a maximum 400 A through ground will be safe side. Then a low resistance of 13.8*1000/sqrt(3)/400=20 ohm and 400 A[for 10 seconds]inserted between neutral and grounding grid, will be fair.
Still a phase-to-phase- to- ground fault could occur and the short-circuit current
will be 1.73/2*I”k=2.9 kA
A grounding conductor of 50 mm^2 will be good even for 10 seconds.
If no 13.8 kV cable or OHLine will leave the substation area no 13.8 kV ground fault current will flow through the ground-only through grounding grid- and then this current will not contribute to the GPR.
At the high voltage side the ground fault current is the result of phase-to-phase-to ground fault-if no grounded neutral is provided. Since the source is on a remote location the grounding resistance on both location has to be taken into consideration.
For high voltage incoming overhead line it is recommended to follow IEEE 80/2013 proceeding in order to design the grounding grid -GPR, the touch and the step voltage for the maximum prospective current and the fault clearing duration-usually 6 cycles[0.1 sec]. has to be less[or equal] to the allowable
 
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Ingenieur

Senior Member
Location
Earth
Get IEEE Guide for Safety in AC Substation Grounding

this is a life safety system
it takes experience
seek consultation

there are standardized designs based on voltages involved and soil resistivity
 
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Smart $

Smart $

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Location
Ohio
anasshad said:
...
Or just give me some clues about where to start. I have to provide IEC and NEC standards for each step.
...
Click to expand...
Welcome. :)

Because the NEC only covers premises wiring, I have to assume this is not a power company transformer and completely under NEC purview.

The secondary is a Separately Derived System. Grounding requirements can be found in 250.30 (that's Article 250, Section 30).

The equipment grounding conductor size run with the primary conductors is determined by the supply overcurrent protection device (OCPD) rating per 250.122.


For a primary or a secondary over 1000V, disregard previous and see Article 250 Part X.
 
petersonra

petersonra

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I wonder how he is planning to connect up the 6700+ Amp secondary of the transformer to his switchgear. Should be some interesting bus work.

do they even make switchgear with bus ampacity in that range?
 
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jim dungar

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PE (Retired) - Power Systems
anasshad said:
I have to provide IEC and NEC standards for each step.
Click to expand...

This sounds like a homework assignment.
IEC and NEC standards are often not in full agreement particularly when it comes to earthing/grounding.
You should be designing to one or the other, not both.
 
Sahib

Sahib

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Location
India
petersonra said:
I wonder how he is planning to connect up the 6700+ Amp secondary of the transformer to his switchgear. Should be some interesting bus work.

do they even make switchgear with bus ampacity in that range?
Click to expand...
Ampacity may not be a problem but breaking capacity a problem IMO.
 
Sahib

Sahib

Senior Member
Location
India
jim dungar said:
This sounds like a homework assignment.
IEC and NEC standards are often not in full agreement particularly when it comes to earthing/grounding.
You should be designing to one or the other, not both.
Click to expand...
I believe harmonisation of ANSI/IEEE and IEC standards underway.
 
Sahib

Sahib

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Location
India
petersonra said:
I wonder how he is planning to connect up the 6700+ Amp secondary of the transformer to his switchgear. Should be some interesting bus work.

do they even make switchgear with bus ampacity in that range?
Click to expand...
Ah! The OP transformer can't be a distribution transformer and so the 6700+ Amp secondary rating not applicable to it. It must have a MV secondary.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
petersonra said:
I wonder how he is planning to connect up the 6700+ Amp secondary of the transformer to his switchgear. Should be some interesting bus work.

do they even make switchgear with bus ampacity in that range?
Click to expand...

Sahib said:
Ah! The OP transformer can't be a distribution transformer and so the 6700+ Amp secondary rating not applicable to it. It must have a MV secondary.
Click to expand...
OP did not state voltages. No way to determine rated current without them.

The only place under NEC purview that would use a 5.6MVA transformer is big industry (TTBOMK).
 
Sahib

Sahib

Senior Member
Location
India
Smart$: My last post is based on the ready availability of maximum size of transformer with LV secondary in the market.
 
R

rian0201

Senior Member
Location
N/A
the question is earthing.. regardless of transformer size, as long as ground fault kA is given, max fault clearing time, persons weight, soil resistivity, and others.. we can calculate.. but if the design is non standard configuration compared to ieee, the only way to solve it, is through finite element..


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