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THE TRANSMISSION GALLERY

TALES FROM A COLD FIELD

Being fairy-stories told to the author as a young engineer
© Ray Cooper, 2005 (2nd revised edition, Jan 2006)

The Juice

Transmitting stations use a lot of power. If that power disappears, a lot of people have blank screens. So the rule is, This Shall Not Happen. To this end, all main transmitting stations in the U.K. have two separate mains feeders, approaching the site from two different directions and fed from different sections of the National Grid. (Actually, there is one exception - it was found impossible to provide two mains feeders for the UHF station at Bilsdale except at disastrous expense. So that station has its own private gas-turbine installation as a standby. There is quite a story behind that, but not for these pages).

These two feeders may be used in different ways, but at Sutton they fed into an automatic changeover system, that in the event of a failure of the feeder in use switched over to the other feeder if the break exceeded about seven seconds. Mains came into the station at 11kV, three phase, and was transformed down to 415V three phase.

Upon the arrival of FM in the 'fifties, the station was also provided with its own diesel-alternator standby generators. These were paid for by the Government, who felt that sound broadcasting stations would be an essential link in the event of any national emergency (nature unspecified) that could cause power disruption sufficient to black out large parts of the National Grid. Whether they considered the fact that the number of battery-operated FM receivers in use at that time was minuscule, is unrecorded.

These diesels were 120kVA units by Paxman, three of them housed in a newly built detached diesel house. They could be run in parallel to provide enough juice to power the complete FM system. In fact they hardly ever did so: testing them in this mode was impossible without causing a break in programme, so what was done was to use two of them to power half of the FM system. Since the diesels were now rather under-run, adding the medium-power reserve 405-line transmitters running into test load made up the shortfall.

They were completely manual in operation. They had to be started manually (no, you didn't have to crank them by hand, they had motor starters, but someone had to push the button), synchronised together manually, load-switched manually and stopped manually. It was a wise idea for somebody to keep an eye on them whilst running, because if one of them dropped off due to some problem - overheating, low oil pressure etc., the remaining one would try to carry on manfully but of course became itself heavily overloaded and usually ended up belching dense black smoke from the exhausts. They wouldn't stand too much of that sort of thing.

Testing them may have been one thing, but it takes a real emergency to show up the flaws in the system. So when, early one morning, the juice went off on both feeders and the station was plunged into darkness (save for the emergency lights - fed from a 110V battery), the night watchman, who was the only person on site at the time, had no option but to rouse the Engineer-in-Charge from his slumbers. Well, the EiC had no problems getting the diesels running, and shortly afterwards such services as should have been, were radiating. That is to say, the transmitters were on - unfortunately, there was no programme modulation on any of them. The subsequent witch-hunt revealed that there was just one panel of amplifiers in the Lines Termination Room that had never been connected to the diesel-maintained supply.

When the PCM link came into operation, its power supply became more critical since not only Sutton but also all points north were dependent on it. A manually started diesel was of little use in the middle of the night, and even in the daytime the Paxmans could take several minutes to be up and running. So a small auto-start Lister diesel-alternator set was provided which powered the PCM SHF links only, and lived in a corner of the diesel house. This would auto-start after about ten seconds if both mains feeders or the changeover system failed.

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mb21 by Mike Brown
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