Lightning protection for ? Systems

[tinfoil]

After consulting with two supppliers and getting three answers, I thought I'd ask the board here:

What is the recommended practice for sizing lightning arrestors for 3-wire overhead and underground distribution systems? The two voltages I have to address are 4800V?, and 26.4kV?.

I have been told to use arrestors with 90%, 100%, and 110% MCOV, w.r.t. line-to-line voltage (Depending on who I asked).


As long as I am on the topic, what should the %MCOV be for grounded 4 wire systems as well, according to the experts?

 

[tinfoil]

Sorry, in the previous post, my 'delta' triangles became ? marks.

 

[dpc]

One critical variable is the system grounding. For a system that will always be solidly grounded, the MCOV should be about 10 to 15% above the nominal line-to-ground voltage.

If the system is ungrounded or grounded through any type of impedance, the recommendation is to make the MCOV 10 to 15% higher than the nominal line-to-line voltage.

So for your delta systems, assuming they are ungrounded, I would use an MCOV of at least 5.3 kV and 29 kV respectively.

(But I'm no expert on surge arresters)

 

[cuky2000]

MCOV rating will depend if the arrester is selected for normal duty or severe duty as well the capability to withstand temporarily overvoltage above the MCOV based on duration and frequency of line-to-ground fault conditions, system resonance and other type of overvoltages.

The enclose info will provide you with a typical range of MCOV close to your application and a generic curves to determine the coefficient of grounding if you input the specific system parameters.

Beware that each manufacturer has different curves for each class of arrester that define the performances of the product. Made sure you speak with a qualified manufacturer rep and not just with a sale person.

Good luck.

 

[jghrist]

One note on cuky's tables: The Ohio Brass table lists MCOV while the Cooper table lists arrester rating. A 6 kV rated arrester has an MCOV of 5.1 kV (with US standards).

I suppose you could get by with less than 5.1 kV MCOV if your protection detected a Ø-grd fault fast enough to stay within the TOV curve for the particular arrester. There are differences among manufacturers in how much time a temporary overvoltage can be withstood. At this voltage, the protective margins are probably more than sufficient to use the 5.1 kV MCOV rated arrester. I wouldn't try to use less on an ungrounded system. The next lowest standard MCOV is 2.55 kV, so the TOV during a Ø-grd fault would be pretty high.

 

[RRaghunath]

The IEEE/IEC definition of "Effectively grounded" system says that the when one of the phases goes to earth the voltage of the healthy phases can rise to a maximum of 80% of rated system phase-ground voltage.
Thus, for effectively earthed system, the MCOV should be 80% of the rated phase-ground voltage.

In case of non-effectively grounded system, the healthy phase (to ground) voltage can equal to phase-phase voltage (when one of the phases goes to ground).
Hence, the MCOV, as a minimum needs to be same as the system rated ph-ph voltage.

In practice, the MCOV and the rated voltage of the LA are chosen after studying the characteristic curves of the LA (the residual voltage especially) and comparing the same with the required protection level, after considering the BIL of the equipment to protected against lightning strikes.

For the reasons cited above, the rated voltage / MCOV of the LA is many a time greater than the rated ph-ph voltage of the system.

Trust the above is helpful.

raghunath_n00@rediffmail.com

 

[rodaxco]

Transformer yard area of dimensons 14x10 open from above and surrounded by wall of 11m height needs lightning protection.Does it need collecting tips(and if yes how many)or hot dip galvanized steel down conductors along the wall are enough?

 

[Xuewei]

Which IEC code describes the requirements for lightning protection?

 

[cuky2000]

Check IEC Std 71-2 "Insulation Coordination"