What is the difference betweem a transmission system and a distribution system? 3

[TransPlanner86]

Is there a good definition somewhere that determines what is the difference between a transmission system and a distribution system?

When I think a distribution system, I usually think of lines coming out of substation that have all of the equipment mounted on poles (transformers and circuit switchers). I am dealing with a 33kV system that has no pole mounted equipment, and customer load and generators are connected via substations. This sounds more like sub-transmission to me. Any thoughts?

 

[DTR2011]

I would say that the difference between distribution, sub-transmission and transmission is determined by voltage level. In ANSI, this cutoff is 60kV for transmission voltages.

http://books.google.com/books?id=Lo...v=onepage&q=ansi transmission voltage&f=false

Distribution voltages would generally be no higher than 34.5 kV and often more likely to be around 15kV. The definitions could change based on what types of loads are in the area as well (industrial vs. residential).

There are many "it depends" to your question. The local (or regional) regulatory board may ultimately define this, as well as how the utility is structured (regulated or de regulated). Many utility companies, for example, are no longer vertically integrated, and have split up into separate transmission or distribution companies.

As far as the pole mounted equipment analogy, I would not necessarily consider that a useful one. There are many installations that utilize pad mounted transformers and associated switchgear at distribution voltages up to 34.5 kV.

 

[cranky108]

NERC can't even tell us what is a transmission system, so how are we supose to know.

In general, a distribution system is radialy fed, and has many distributive taps. The lower voltage idea, is a dividing line that has no clear level. But generally in the US, 15 kV class and lower is not transmission, and 69 kV and above is not distribution. What is inbetween is anybody's guess.

 

[oldfieldguy]

"But generally in the US, 15 kV class and lower is not transmission, and 69 kV and above is not distribution. What is inbetween is anybody's guess."

Two of the big utilities I worked for called 69kV and 45kV "Subtransmission".

old field guy

 

[davidbeach]

What is inbetween is anybody's guess.

And that's why we have 24kV transmission and 35kV distribution.

 

[mesutphen]

I found this in the Standard Handbook for Electrical Engineers:

Distribution Systems: "Broadly speaking, 'distribution' includes all parts of an electric utility system between bulk power sources and the consumers service-entrance equipments... In general, a typical distribution system consists of (1) subtransmission circuits with voltage ratings usually between 12.47 and 245kV which deliver energy to the distribution substations; (2) distribution substations which convert the energy to a lower 'primary system' voltage for local distribution and usually include facilities for voltage regulation of the primary voltage; (3) primary circuits or 'feeders,' usually operating in the range of 4.16 to 34.5kV and supplying the load in a well-defined geographical area; (4) distribution transformers in ratings from 10 to 2500kVA which may be installed on poles, grade-level pads, or in underground vaults near the consumers and transform the primary voltages to utilization voltages; (5) secondary circuits at utilization voltage which carry the energy from the distribution transformer along the street or rear-lot lines; and (6) service drops which deliver the energy from the secondary to the user's service-entrance equipment."

Transmission Systems: "The function of the transmission system is the transmission of bulk power to load centers and large industrial users beyond the economical service range of the regular primary distribution lines."

 

[pwrtran]

Transmission system - needs step-down transformer station to further reduce the voltage to distribution level. Like highways
Distribution system - step-down transformers convert the voltage to end user's level, 120V, 240V, 208, or 600V. Like local roads or county roads

 

[odlanor]

a distribution system is a radial system.

 

[pwrtran]

Not always true. Some weak transmission is radial feed and some strong distribution is dual feed.

 

[davidbeach]

Distribution is the portion of the system where the typical customer is served through one last utity owned transformer. Transmission is the portion of the system where there are at least two utility owned transformers between there and the typical customer. Exceptions abound.

 

[cranky108]

Distribution as in the name, is used to distribute energy to customers. Transmission as in the nams, is used to transmit energy over some distance.

So what do you call a 7 kV delta circuit connected with one source, and has two generation sources, and three energy consumers?

 

[davidbeach]

A mess.

 

[cranky108]

Thank you david. We call it one of a kind.

Why is a gas distribution system called distribution? Or gas transmission called transmission? It's the same concept, except we use voltage level, not pressure in a pipe.

In the gas world they have a collector system, which we don't normally have (except renewables) because we have large generators connected directly to the transmission.

 

[davidbeach]

One of a kind.

As never again or over my dead body?

 

[cranky108]

As in a rare set of conditions we don't see happening again.

Mainly small hydros, care takers houses, and a 911 relay tower. Very sparce.

 

[cuky2000]

To understand this issue perhaps we need to study the history of the utilities in the US.

In the US there are approximately 3,273 traditional electric utilities and around 1,738 nonutility power producers ranging from investor-owned, publicly-owned, cooperatives, and Federal/ Authorities (TVA, ACOE, Bureau of Reclamation, NYPA, BPA, etc). In few decades ago NERC grouped all utility in 8 regional organizations (PJM, WECC, NPCC, FRCC, MRO, etc).

It is not a surprise in the US there is not agreement to classifying distribution and transmission voltages since each utility is an independent entity many of then in operation long history before regulating agencies.

Even at the distribution level today there is several system voltages variation that survives after many years in operation such as 7.2/12.47-kV, 7.6/13.2kV, 7.9/13.8-kV, etc. In the pass, not only voltage levels were the issue but also different operating frequencies including DC. (NOTE: DC distribution services in NY City was suspended a few years ago)

 

[cranky108]

No more Dc services? How will I charge my cell phone batteries?

 

[Overvoltage]

The only legal way is using a balloon and a cat...

 

[cuky2000]

I am kind of agreeing that it is a mess. Beside “balloon and cat” either legal or illegal, may be another way to get the cell phone charged: What about wireless power from low cost power generation located oversee?

As preposterous as it may sound, it is never-the-less very true as the utility retired the DC service for more than 4,500 customers in NY City 5 years ago. See the press Release below.

http://www.coned.com/newsroom/news/pr20071115.asp

 

[cranky108]

Next thing is they will end two phase service.

Some small towns still use 4160, or 2400 V for distribution.