Definition of Substation 3

[SooryaShrestha]

Sir,
May I put in the forum the following questions?
a) Does IEEE or other standards define as to which substation come under the category of Distribution Substation?
b) I think IEEE standard in one of the pages specified the highest acceptable grounding resistance to be 5 Ohm for Distribution Substation (reason requiring the above definition). If that is true, what are the allowable values of grounding resistance for other substations? As we try to maintain the ground resistance below 1 Ohm, we find sometomes not possible because of limitation of area because of prohibitive cost increase. Which standard in the world specifies the required values of ground resistance?
c) I have made some of the interesting notes in the execution of Grounding. I am shortly preparing the paper and like to send to Eng-Tips.
Thank you in advance.
Soorya B Shrestha

 

[jbartos]

Comment: Reference:
1. IEEE Std 100-2000 "The Authoritative Dictionary of IEEE Standard Terms," Seventh Edition
Reference 1 defines "Distribution Station (Power Operation)" as a transforming station where the transmission is linked to the distribution system.
Reference 1 defines "Substation" in four different ways:
(1) (generating stations electric power system) as an area or group of equipment containing switches, circuit breakers, buses, and transformers for switching power circuits and to transform power from one voltage to another or from one system to another
(2) An assemblage of equipment for purposes other than generation or utilization, through which electric energy in bulk is passed for purpose of switching or modifying its characteristics. Service equipment, distribution transformer installations, or other minor distribution or transmission equipment are not classified as substations. Note: A substation is of such size or complexity that it incorporates one or more buses, multiplicity of circuit breakers, and usually is either sole receiving point of commonly more than one supply circuit, or it sectionalizes the transmission circuits passing through it by means of circuit breakers.
(3)...
(4)...
The term "Distribution Substation" could fall under "Distribution Station" or "Substation." It appears to be an ambiguous term not defined in Reference 1.
(b) There appears to be a wording inconsistency. One needs a proper interpretation to it.
(c) This is by your prerogative.

 

[SooryaShrestha]

In IEEE Std. 80-1986 page 81 no. 12 Evaluation of Ground Resistance on 12,1 it states" Usual Requirements. An ideal grounding should provide a near zero resistance to remote earth. In .....For most transmission and other large substations, the ground resistance should be about 1 Ohm or less. In smaller distribution substations the usually acceptable range is from 1 - 5 Ohm, depending on the local conditions." This shows a definition of distribution substation is required, as this is going to change the requirement of the ground resistance from less than 1 Ohm to 1 - 5 Ohm. So a definition of distribution substation is required. I think my question requires clearance.

 

[jbartos]

Suggestions:
1. IEEE Std 80-1986 is replaced by
80-2000 IEEE Guide for Safety in AC Substation Grounding
Print 200 pages [0-7381-1926-1] [SH94807-NYF] $75.00 * IEEE Mbr: $60.00
PDF: [0-7381-1927-X] [SS94807-NYF] $113.00 * IEEE Mbr: $90.00
2. Going into the retrospect, IEEE Std 100-1984, Third Edition, defines, what could be viewed as a "relevant term," a "Distribution System" as "that portion of an electric system which delivers electric energy from transformation points on the transmission, or bulk power system to the consumers." It appears that the distribution substation could fit inside of the "Distribution System." However, there is no definition of "Distribution Substation" in IEEE Std 100-1984.

 

[GRPatel]

Soorya are you 74 batch from MSU?

 

[redtrumpet]

I'm no expert, but I believe most utilities use 25 kV as their cutoff limit for a distribution line. So a distribution substation could alternatively be thought of as one with a secondary voltage of 25 kV or below.

I don't think IEEE 80 should be used to derive hard and fast rules as a substitute for detailed analysis. I believe IEEE 80 states that substations with very high ground resistances can be made safe through careful design. If the GPR, mesh and touch potentials, and transferred potentials are safe for people and don't interfere with communication circuits, then the ground resistance is largely immaterial. I think what IEEE is saying is that you will have a tough time meeting these criteria if your resistance is above the 1 to 5 ohm range.

 

[redtrumpet]

One other thought - since soil resistivity and ground grid area are the two most important variables controlling the substation ground resistance, in general smaller distribution substations have a smaller surface area and hence a higher ground resistance than larger substations in similar soil.

Rereading your original post, you appear to recognize this fact and thus have answered what you really need to know. The definition of distribution substation is, I think, irrelevant to your problem and just confusing the situation for you.

 

[redtrumpet]

Okay, notwithstanding my above post, here is the definition of distribution substation from the oft-referenced (in this forum) RUS Bulletin 1724E-300:

"A distribution substation is a combination of switching, controlling, and voltage step-down equipment arranged to reduce subtransmission voltage to primary distribution voltage for residential, farm, commercial, and industrial loads."

"Rural distribution substation capacities vary. Substations generally include one 1.5 MVA to three 5 MVA transformers. These substations may be supplied radially, tapped from a subtransmission line, or may have two sources of supply. Most cooperatives's substations have 12,470Y/7200-volt or 24,490Y/14,400-volt distribution circuits."

And I think the last sentence in the third paragraph of the section is telling:

"Confirmation of the technical terms being used is likely to be required since electrical engineers in differing industries may use the same terms to describe similar, yet technically different, criteria."

A line that is applicable to every second thread on Eng-Tips, it seems.

 

[SooryaShrestha]

After long time of absence from eng-tips, I find helpful stuffs I was missing.Thanks Redtrumpet who has given very helpful information.Deserve a star.
GRpatel, yes I am 74 batch from MSU.

 

[GRPatel]

surya,
I have been trying to reach you. You can contact me at patelgunvantray@hotmail.com