IT earthing on MV and HV systems
IT earthing on MV and HV systems
(OP)
Why is IT earthing the chosen method for MV and HV systems?
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IT earthing on MV and HV systems
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RE: IT earthing on MV and HV systems
RE: IT earthing on MV and HV systems
The 'T' is from the Latin word 'Terra' meaning 'earth'. 'I' represents either impedance or isolated. I don't like the duality of the term 'IT' but I didn't make the rules. I don't recall seeing it used much in relation to MV and HV systems, and low voltage IT systems are pretty unusual in the UK.
IT systems are used where there are no line-neutral loads because they provide a means of keeping the prospective earth fault level within reasonable bounds in the case of an impedance-earthed source or reducing it to virtually zero in the case of an isolated system, thus limiting the scale of damage in the event of an earth fault. Impedance-earthed systems are common in industrial plants with a lot of rotating plant because a stator earth fault with a solidly grounded system will usually mean a write-off or major rebuild whereas a similar fault with an IT system usually results in a salvageable motor.
I'll let the utility guys talk about ungrounded systems and reasons why they are used or avoided.
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If we learn from our mistakes I'm getting a great education!
RE: IT earthing on MV and HV systems
So, in answer to the original question, generally they aren't used except for industrial MV systems.
RE: IT earthing on MV and HV systems
Alan
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"It's always fun to do the impossible." - Walt Disney
RE: IT earthing on MV and HV systems
The low voltage 415/240V system is solidly earthed. The 11kV system is either solid or low impedance earthed via a resistor,reactor or petersen coil, the 33kV system is low impedance earthed, and the 132kV (and above)system is solidly earthed.
I'm not sure therefore why oconn thinks IT earthing is the chosen method.
Regards
Marmite
RE: IT earthing on MV and HV systems
In North America, a significant proponent of impedance grounding for medium-voltage systems in the petrochem industry is John P Nelson/Nelson Electrical Engineering. See www.neiengineering.com/pdfs/paper7JN.pdf and www.neiengineering.com/papers.html
RE: IT earthing on MV and HV systems
This is to minimise stator core damage possibility during motor earth faults.
RE: IT earthing on MV and HV systems
In additional to all right answers:
I would recommend read a great SEL docoment
http://tesla.selinc.com/techpprs/6123.pdf.
Shrotly: HV, above 110kV is solidly grounded, MV ( according to SEL document), but in industries MV is (99% of cases) low resistance grounding.
term IT, I see first time for the HV, MV application.
As Marmite wrote, 400/230V is usually also solidly grounded , except a (for example) medical application.
BTW, if I remember right, 132kV with low-impedance grounding (zig-zag trafo) used in Brazil.
Best Regards.
Slava
RE: IT earthing on MV and HV systems
Just for your information, in our grid (Chesf, Northeast of Brazil), all 500kV, 230kV, 138kV systems are solidly grounded. But our 69kV system is supplyed from a delta winding conneted tranformer and there is a zig-zag reactor in each 69kV busbar for grounding reference.
Remember that the "guard cable" or "earth wire" which is installed at the top of the trasmission line towers for lightning protection, connects the grounding mesh of system substations and seems to work as a (almost) "neutral wire" for HV grounded systems.
Best regards,
Herivelto Bronzeado
RE: IT earthing on MV and HV systems
Networks above 35kV (that is 110kV, 220kV, 330kV etc) are so-called "effectively-grounded" - i.e. they are grounded via small impedance, almost solidly.
Networks below 1kV (that is 690/400V and 400V/230V) are normally solidly grounded.
That's what we have in Russia.