Questions on electrical distribution system and motor efficiency

[M. Hamill]

I'm an engineer albeit not an E.E., and I have a few questions that some of
you might be able to answer:

- What is the typical efficiency of the power distribution system as it
applies to typical residential customers with 120/240 VAC? (I ask this
because it seems to me that it must be substantially below 100%, when one
considers that there are 5 or more transformers between the power's company
generation station, and end users; plus impedance losses, etc.) Or,
rephrased, for every 100 kilowatt-hours used at a home, how many
kilowatt-hours would be required at the generating plant's high-voltage
connection to the distribution system?

- What is the approximate efficiency of the best DC motors available these
days (power available to motor divided by mechanical equivalent of
electrical energy used?) Also, what is a more typical efficiency for DC
motors?

- Can DC motors be run more efficiently if there are designed to run at
higher voltages (compared to lower voltage DC motors?)

Thanks in advance,

M.H.

 

[Don Kelly]

----------------------------

I'm an engineer albeit not an E.E., and I have a few questions that some
of you might be able to answer:

- What is the typical efficiency of the power distribution system as it
applies to typical residential customers with 120/240 VAC? (I ask this
because it seems to me that it must be substantially below 100%, when one
considers that there are 5 or more transformers between the power's
company generation station, and end users; plus impedance losses, etc.)
Or, rephrased, for every 100 kilowatt-hours used at a home, how many
kilowatt-hours would be required at the generating plant's high-voltage
connection to the distribution system?

- What is the approximate efficiency of the best DC motors available these
days (power available to motor divided by mechanical equivalent of
electrical energy used?) Also, what is a more typical efficiency for DC
motors?

- Can DC motors be run more efficiently if there are designed to run at
higher voltages (compared to lower voltage DC motors?)

Thanks in advance,

M.H.

Bill has answered your question well and BFoelsch has provided an overall
coal to useful energy to the user, comparison.

As for DC motors, while high voltage DC machines can be built, a practical
limit would be in the 2KV range and an economical limit would be far
lower-say 500V. Compared to the typical AC induction motor, a DC motor is
costlier to build, less efficient in general (possibly by 3 to 5 % ) and
much more difficult to maintain. The commutator/brush system is the main
problem and the power winding is fed through these moving contacts. In
contrast the induction motor has a stationary power winding and no moving
contacts as well as a very simple and rugged rotor. The machine then can be
economically built for much higher voltages and powers than DC motors.
DC motors are better suited to mobile and many specialty applications but AC
machines are the general workhorses. --

Don Kelly [email protected]
remove the X to answer

 

[Beachcomber]

\
All of the previous posters have provided accurate answers to your
questions. One thing that was not mentioned however, when discussing
the overall efficiency of a power system is the heat loss of the
transmission lines themselves.

You mention distribution systems in your post, but if you are
including the 5 or more transformers that are part of the overall
system, this would include both transmission and distribution.

Some of the transmission lines in certain parts of the country are
overloaded and run HOT (100C) which is the boiling point of water and
the thermal limit of many of the lines.

Also, at the turn of the previous century, a high voltage dc
transmission system was developed using series connected DC motors and
generators. It was primitive, but relatively efficient, and was used
until the 1930's in Italy.

Look up the Thury System of DC Transmission at:

http://en.wikipedia.org/wiki/HVDC

for more information.

Beachcomber

 

[Don Kelly]

The heat loss of transmission lines was included in the loss/efficiency
data. Sure many lines run hot , depending on many things including
direction of the line, wind, thermal insolation, etc. 100 degrees is
tolerable but will result in decreased conductor life.
The Thury system died because it was expensive to maintain and it was
definitely not "relatively efficient" compared to a corresponding AC line
at the same distance, voltage and power transfer .
Quote from your reference:" Other Thury systems operating at up to 100 kV DC
operated up until the 1930s, but the rotating machinery required high
maintenance and had high energy loss."

It served a need at the time but the MG set system was never really viable.

 

[Beachcomber]

The heat loss of transmission lines was included in the loss/efficiency
data. Sure many lines run hot , depending on many things including
direction of the line, wind, thermal insolation, etc. 100 degrees is
tolerable but will result in decreased conductor life.
The Thury system died because it was expensive to maintain and it was
definitely not "relatively efficient" compared to a corresponding AC line
at the same distance, voltage and power transfer .
Quote from your reference:" Other Thury systems operating at up to 100 kV DC
operated up until the 1930s, but the rotating machinery required high
maintenance and had high energy loss."

It served a need at the time but the MG set system was never really viable.
--

Don Kelly [email protected]
remove the X to answer
----------------------------

Thanks Don:

Perhaps, I should have said relative to available technology at the
time for comparative AC long distance transmission systems. Else why
would they invest in the DC technology in the first place?

MG sets were tried on an experimental basis (early 1900's) for
substations in Chicago by Mr. Insull's Public Service Company (later
Commonwealth Edison) in central city locations where DC was used by
elevators. New York, I believe, had exclusively DC power districts
in the central city up until the 1950's from the vintage ads I've seen
for special radios that would run on this type of current.

MG sets were certainly viable for electric traction systems (such as
the Chicago Transit Authority's 600 VDC supply) and the Interurban
railroads of the period, and also the New York Transit Authority until
well past the mid-century mark, when mercury-arc rectifiers succeeded
as replacements.

Beachcomber



Beachcomber

 

[M. Hamill]

What's a more impressive number is the "utilization" efficiency, which
is to say the usable output from electrical devices divided by the
total energy input at the power plant.

Last data I have (year 2000) puts that at about 17%. This figure
reflects all the inefficiencies, boilers, turbines, auxiliaries etc.
as well as the inefficiencies at the user's end; incandescent lamps,
etc.

In other words, on the average, it takes 100 BTU of coal to deliver 17
BTU of ultimately useful energy.

----
Thanks for your post.

I really don't know about the efficiencies of electrical distribution
and electricity use, but I used to work with a power company that had
most of its generation capacity in coal-fired power plants, which is
the norm in the U.S. And this company's most efficient coal-fired
plants had a "heat rate" of about 9900 BTU/ kilowatt-hr. (I'm going on
memory here.) When one factors in the conversion of 3413 BTU per kw-hr.,
that translates to a "thermal efficiency" at the power plant of about
34.5%, or, about 100 BTU of coal to deliver 34.5 BTU at the power's
company's generator step-up transformer connection to the power grid.

M.H.

 

[Don Kelly]

----------------------------

Thanks Don:

Perhaps, I should have said relative to available technology at the
time for comparative AC long distance transmission systems. Else why
would they invest in the DC technology in the first place?

MG sets were tried on an experimental basis (early 1900's) for
substations in Chicago by Mr. Insull's Public Service Company (later
Commonwealth Edison) in central city locations where DC was used by
elevators. New York, I believe, had exclusively DC power districts
in the central city up until the 1950's from the vintage ads I've seen
for special radios that would run on this type of current.

MG sets were certainly viable for electric traction systems (such as
the Chicago Transit Authority's 600 VDC supply) and the Interurban
railroads of the period, and also the New York Transit Authority until
well past the mid-century mark, when mercury-arc rectifiers succeeded
as replacements.

Beachcomber

Experimentally to try to get the advantages of higher voltage DC
transmission which they could see. However, prior to electronic inverters,
the dream was really not feasible, either economically or otherwise. For
10KV at 2KV per machine, there would be 10 stages(up and down) so the
efficiency would be leas than 40% excluding line losses, and maintenance
costs would be horrible. Even now, DC transmission would not be normally be
used for such short, low power links as it would not be cost competitive. I
say normally because there are specific cases where it is viable - such as
some underwater links, ties between two systems at different frequencies or
as a weak tie between two large systems at nominally the same frequency
where a weak AC link may hold only for a short time before becoming
unstable.

The use of MG sets for specific applications such as those you mention was
common and for these applications, was reasonably efficient and economical
in comparison to the Thury scheme. My comment about MG sets dealt with them
as used in the Thury system. In Central New York there were regions
supplied by Edison's original DC system and rather than go in for a
wholesale rebuilding of the distribution system, an MG set was an attractive
alternative (for a while). Whether they lasted until the 50's - I don't
know as the original infrastructure would have been inadequate by then.

 

[Bud--]

The heat loss of transmission lines was included in the loss/efficiency
data. Sure many lines run hot , depending on many things including
direction of the line, wind, thermal insolation, etc. 100 degrees is
tolerable but will result in decreased conductor life.

Why does it reduce the conductor life?

bud--