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Old 20th Sep 2018, 2:36 pm   #61
julie_m
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Default Re: Last public DC mains supply in the UK?

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Originally Posted by broadgage View Post
The terminals connected to the "start" of each of the 3 LV windings are identified as red, yellow, and blue in the usual way.
The other 3 terminals connected to the "finish" of the relevant windings are usually in written documents referred to as red* yellow* and blue*
Not "green", "violet" and "orange" then?
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Old 20th Sep 2018, 2:47 pm   #62
broadgage
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Default Re: Last public DC mains supply in the UK?

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It's always interesting to learn about non-standard electricity supplies.
Yes, and they are getting less common "every opportunity should be taken to replace legacy systems with standard 3 phase, 4 wire, at 230/400 volts"

"no significant expansion of legacy systems is permitted"

The main drawbacks of such systems is the risk of a relatively prolonged outage in the event of a transformer failure.
The failure of a "conventional" substation transformer is normally handled by backfeeding at LV from adjacent mains on different transformers.
This can be done within an hour by two men in a van inserting fuses or links into street link boxes.
If this can't be done, then a large transportable generator may be connected to the LV busbars in the substation. This can usually be done within three hours.

The odd legacy systems can't be paralleled, nor supplied from a large generator. The supply would remain off until the new transformer was delivered. A small stock is kept of both reverse polarity and diametric transformers. But transport from a distant stores department and crane hire is unlikely to be achieved in less than 12 hours.
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Old 20th Sep 2018, 6:19 pm   #63
ex 2 Base
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Default Re: Last public DC mains supply in the UK?

A bit off topic. When I worked for BR in 1950's they had a large warehouse with 3 wire DC. 460 volts for the motors and for the lighting 230 volts positive to neutral or negative to neutral. All supplied from two transformers and two large mercury arc rectifiers. The line shafting motors were open brush gear type with oil rings for lubrication and I think made by Siemens dated about 1910.I can't ever remember the commutators needing skimming, the motors just purred away.Originally the local power station supplied the DC. Oh, I forgot the Face plate starters. I didn't know about the reverse connected transformer, very interesting. Ted
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Old 20th Sep 2018, 8:58 pm   #64
hannahs radios
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Default Re: Last public DC mains supply in the UK?

broadgage thanks for the explanation of the system used, as I said I couldn't remember the details but did remember about the two transformers used. My mate told me that in summer the voltage can be up to 260 volts and in the winter down to 220. I think the voltage regulation's not too good.
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Old 21st Sep 2018, 3:44 am   #65
Synchrodyne
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Default Re: Last public DC mains supply in the UK?

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Quote:
Originally Posted by Synchrodyne View Post
...railway rheostatic braking used either separate excitation of series wound motors, or self-excited series-wound motors... trolleybuses with rheostatic or regenerative braking used compound-wound motors, tramcars with regenerative braking used compound-wound motors, late practice for tramcars (including the PCC type) with rheostatic braking was self-excited series.
Thanks for informative post and supporting literature Steve! So basically with the series-wound DC motors, auxiliary coils were completely necessary for regenerative braking. Makes sense!
No problem, but I suspect that in what I said I sacrified preciison for concision. In the railway case, separate excitation of series-wound motors for electric braking purposes was achieved by using the existing series field for this purpose, power-brake changeover switches (somewhat like reverser switches) being used to separate the field circuits from the armature circuits. Additional field coils were rare in railway DC traction motors.

More generally, re the use of existent DC three-wire distribution systems for AC, I imagine that this could explain the origins of the standard American three-wire system of single-phase AC distribution. I suppose that one probably inadvertent benefit of the three-wire system would be that, assuming a statistical distribution of appliances on each side, there would be much reduced net DC on the transformers secondaries from the use of AC-DC radio and TV receivers.


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Old 21st Sep 2018, 4:44 am   #66
Synchrodyne
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Default Re: Last public DC mains supply in the UK?

The list of UK electrical supply voltages, etc., at the rear of the Electrical Engineerís Reference Book ninth edition, 1958 shows some locations as also having some two-phase distribution. Exeter was one example. Possibly this came about through re-use of existing three-wire DC distribution systems. In that case the neutral (common) would have been nominally undersized, although perhaps considered satisfactory for typical loads. Conversion from 3- to 2-phase was easily done with Scott tee-connected transformers.

By way of broad comparison , here in New Zealand, standardization of electrical distribution came quite early, with most conversions (from DC and non-standard AC) done in the 1920s and 1930s. But some DC systems lasted until after WWII.

The decision to standardize on 50 Hz, 3-phase, 4-wire, multiple-earth-neutral (MEN) type distribution was made in 1920. Part of the MEN system was that at each consumer switchbox, the neutral was bonded to local earth by the MEN link, so I think we avoided some of the earthing issues that have cropped up elsewhere about differences between local earth and supply earth. Shortly after that decision, the voltage was standardized at 230/400 volts. There was also early use of 11 kV for primary distribution; for example Christchurch started building a city-wide underground 11 kV network in 1913. There was also early use of direct transformation from 11 kV to 230/400 volts, without an intermediate 3.3 or 6.6 kV step. A very early such installation by Lloyd Mandeno (later of SWER fame) would have run fairly close by to where this is being written.

Even so, special-purpose DC systems were retained in Christchurch and Wellington additional to the standard AC systems until the later 1950s. These were said to be for applications such as elevators and dentist drills. Auckland though had a fairly extensive inner city DC system, 3-wire 230/460 volts. Conversion had not been completed before WWII, and something around 100 route miles still remained, with slow progress resumed after WWII. The work was finally completed in the late 1950s. Whether the three-wire distribution system was re-used for AC I donít know. (This kind of information is not so easy to find.) There were some cases of 230/460 volt three-wire single-phase distribution in rural areas, but these were at the end of two-wire 11 kV single-phase or SWER spurs.

Notwithstanding the survival of the Auckland DC system into the 1950s, I have not found any evidence that the local setmakers offered AC-DC type radio receivers for use in the DC area in this period. Their products of this period seem to have been AC-only, and were fitted with 3-core power leads, i.e. true Class I by later terminology. Perhaps those living in the DC area used battery receivers. Post-WWII, the DC system had a known limited life, with progressive diminution, so production of special appliances was probably seen as something best avoided. Whether live chassis AC-DC equipment would have met the safety regulations of the time is debatable, and the supply authorities didnít like them anyway, so they would not have been keen on any such receivers being retained in use after conversion to AC.


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Old 21st Sep 2018, 1:18 pm   #67
Bill
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Default Re: Last public DC mains supply in the UK?

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According to the Ian Allan "British Rail Handbook" for 1981, the Class 76 locos for the 1500VDC Manchester - Sheffield - Wath services had regenerative braking. It says " These locomotives were fitted from the first for regenerative braking, and the driver had to balance the regenerated voltage with the voltage in the overhead line before allowing regeneration to begin." Rheostatic braking was added later, integrated with the normal braking operation, and only required the operation of a switch to activate it. No other technical details given.
The Woodhead railway is relevant to this topic on two counts.
The regenerative braking was meant to help power trains uphill, unfortunately there was often not enough traffic for that and the re-generated power had to be dissipated in load banks (the right term?) at Hadfield substation. There were problems of excessive voltage on the overhead lines causing circuit breakers to trip as well as some burning out of resistances on the locos themselves.
Secondly, when the electrification was done in the early 50's a few houses for railway staff were built at Woodhead Tunnel itself. These houses were powered from the 1500v dc traction supply via a rotary converter, don't know its output voltage, perhaps 200v? The line closed in 1981 so I assume the houses got connected to the National Grid then.Ironically , the " new" Woodhead tunnel built 1954, now carries HV cables (132KV?) instead of railway.
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Old 21st Sep 2018, 2:17 pm   #68
Dai Corner
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Default Re: Last public DC mains supply in the UK?

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Ironically , the " new" Woodhead tunnel built 1954, now carries HV cables (132KV?) instead of railway.
Technology and economics (and perhaps a bit of politics, but let's avoid that!) dictate that it's now cheaper to transport energy across the Pennines as electricity than as lumps of coal.
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Old 21st Sep 2018, 3:11 pm   #69
broadgage
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Default Re: Last public DC mains supply in the UK?

One of the original reasons for building the national grid was to facilitate the burning of coal in large modern, high efficiency power stations situated in the coal fields.

It was indeed cheaper to transmit electricity by high voltage grid lines, than to transport coal to power stations nearer the load.
Electrical losses were said to be 1%.
Coal losses in extra handling, and the coal burnt by steam locomotives to haul the coal were said to be 2%.
A well built grid line was less vulnerable to bad weather than a railway.
OTOH, a railway built primarily to transport coal was also useful for other traffic.

All a bit academic these days as coal is much less used.
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Old 21st Sep 2018, 3:19 pm   #70
broadgage
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Default Re: Last public DC mains supply in the UK?

Re post #57.
The railway houses built near Woodhead tunnel were I suspect supplied with 240 volts AC. By the 1950s, DC was on the way out for new installations, as distinct from continued use of existing infrastructure.

A rotary converter run "inverted" from the 1500VDC traction current would have supplied this.
An AC supply, probably from the same rotary would also have been desirable for signalling.
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