Household DC Mains Supplies

[broadgage]

NYC did realy have DC distribution until 2007.
Towards the end, this supplied only a handful of consumers, as a concerted effort had been made to change to AC.
IIRC, as recently as 1999 there were many hundreds of DC customers.

The main demand was older office buildings that had DC lifts and other plant.
Older theatres still had DC machinery, and until recently carbon arc follow spots.

I very much doubt that many customers had ONLY DC service, they would have had AC as well to facilitate use of modern appliances.

The ending of DC service caused howls of outrage ! "we have only had that auditorium extract fan for 100 years. whatya mean we have to buy a new one"
And "this elevator was installed personally by Mr Otis, why should we have to replace it ?"

[dseymo1]

To which the response presumably was "You don't have to - rectification plant is relatively small and cheap these days".

[Peter.N.]

Very interesting thread. In the '50s as part of my job as a TV engineer (we repaired anything in those days) I used to maintain the PA system in a paper mill near Orpington, there was no health and saftey then of course so I had the complete run of the place.

It was very old and was equipped with many different types of motors including many DC types, these were powered by a huge rotary converter, certainly the biggest I have ever seen, it was housed in the boiler house and must have been something like 10' long and about 5' square, it ran at what looked like a ridiculously low speed and the brushes looked something like 6x4". It was fed directly from the grid by its own oil cooled transformer - substation size. I think it was sadly demolished in the '60s.

Peter

[broadgage]

Originally Posted by dseymo1

To which the response presumably was "You don't have to - rectification plant is relatively small and cheap these days".

That was certainly the view taken by the power company.
However in practice conversion of AC into good quality DC was far from easy.

The obvious approach would be a three phase, six pulse solid state rectifier and this certainly worked to an extent. A transformer would almost certainly be needed, which cost money, took up space, introduced losses, and formed a new single point of failure.
Unless the transformer and rectifier were very over sized, such an arrangement coped poorly with large starting currents.
A 200 ampere DC service should start a motor with a running current of 100 amps, and a starting current of 600 amps. A 200 amp rectifier and transformer would not.
A six pulse rectifier introduced undesirable ripple, and smoothing resulted in the voltage varying even more according to the load.

A 6 phase, 12 pulse rectifier was an improvement but required a special and expensive transformer.

Also some DC loads briefly fed power back into the DC mains, this was easily accepted and caused no problem, but no common transformer and rectifier would work thus.

[Synchrodyne]

Originally Posted by Neil Purling

Synchrodyne: Was it a typo or did NYC really have some DC distribution until 2007?
If true I assume it was for industrial or transportation & therefore off-topic, but I was curious.

Cunningham (op. cit.) gave the exact DC switch-off time as 2007 November 14, 14:42 local time. He also said: “The severing of the last cable marked the end of 125 years, two months, nine days, 23 hours and 42 minutes of direct-current power sales since Thomas Edison closed the switches at Pearl Street.”

Apparently there were still around 5000 DC customers in NYC in 2000, after which a concerted effort was made to either convert them to AC or install rectifiers on their premises. A lot of the remaining DC load was older elevator systems, which might have been in residential as well as in commercial buildings. Whether any domestic load remained is unknown; it seems unlikely, but not impossible, bearing in mind that the DC distribution system was still essentially the early 110/220-volt 3-wire system (although I think 120/240-volt in its later days), intended for both commercial/industrial and domestic loads (but not transportation loads, such as the NY subway, which had their own conversion stations.)

I recall reading a while back that San Francisco city retained a DC distribution until quite late, perhaps even later than NYC, although right now I cannot find the reference. Again it was mostly for older elevator systems in its last years.


Cheers,

[John Caswell]

In my servicing days 1958-73 in W London there was still a lot of DC in the Hammersmith/Fulham area. If we had any calls there it was always a 2 man job, just in case the relevant engineer got "caught" by the DC power then the other could knock him free. I also accidentally got "caught" by a set when shifting it around on the bench as the power switch had failed and I got my body across the HT supply directly on the bridge rectifier. Prompt action by my colleague saved the day and me. Not very pleasant!!

John

[broadgage]

To add to the points already made, there STILL areas of London with legacy cable systems that date back to DC days.
The present day supply is of course AC, but non standard in various ways so as to best utilise mains intended for 3 wire DC.

The simple way is to connect two phases and the neutral to a 3 wire main. This works but finds little favour due to the continuous neutral or return current causing heating and voltage drop.

Other alternatives include use of 6 phase transformers, with individual 3 wire mains connected so as to give 240 volts phase to neutral and 480 volts phase to phase.
Alternatively, the substation contains a pair of transformers, one of which has reversed polarity. A new 4 core main is run in a ring around the area to be served which is divided into 3 zones, red, yellow, and blue.
In the red zone, the red core of this cable is cut and jointed to all the existing 3 wire mains, one outer from each main being connected to the clockwise side of the ring and the other outer being connected to the anti-clockwise part of the ring, the center wires of the old mains being jointed to the black neutral core of the new cable. In the red zone, no connection is made to the yellow or blue cores of the new cable.
Existing 3 wire mains in the yellow or blue zones are connected similarly to the corresponding cores in the new ring.

Note that single phase for domestic and similar purposes is easily supplied and that the customers knows nothing of these odd arrangements.
3 phase is not available without a new cable from the substation.

Legacy phase colours used for clarity.

[hannahs radios]

Wow broadgage that sounds like a complex way to do it I had to read it twice to understand. Weymouth has a small substation in town with two transformers I wonder if this is working on the system you describe? It's in the very old part of town. I'll try to find out. I never gave twin transformers much of a thought until now. I'm told that the voltage regulation is poor in that area varying between 210 and 265 volts depending on time of day.

[richard.cs]

If you're interested in the various legacy systems take a look at section 7.5 of this document (pages 15-17):
http://library.ukpowernetworks.co.uk...n+Standard.pdf

These is a diagram of the "phase-antiphase" system on page 16.

Regarding Weymouth the poor voltage regulation perhaps suggests 2-phase cables with undersized neutrals being supplied two phases at 120 degrees, the neutrals would then run heavily loaded and significant voltage variation could be seen.

[emeritus]

265V is significantly in excess of what I understand is the legal limit of 253V ( 230V + 10%)!

[hannahs radios]

Thanks for the link I will have a good read of that it seems full of useful information. The 265 volts I quoted may be a bit high it comes from second hand information so may not be completely correct.

[paulsherwin]

265V is not unknown but would be very unusual in an urban area nowadays. It's a fault condition of course, and the electricity supplier would be expected to fix it.

[Refugee]

That reading might have been taken with a cheap meter close to a noisy SMPSU.

[colly0410]

Richard.CS... Thanks for the link, found it very interesting...

[hannahs radios]

The 265 volts reading was taken with an ordinary multimeter of good quality the shop where it was done had only one SMPSU in a portable TV the rest of the load was from pre heat fluorescent fittings and an ancient heater. The wiring in the shop dates back to the 1930s even the fusebox is of the that age. When the owner comes back I'll ask him