London Banter - View Single Post

robsignals

(Richard Catlow) wrote in message om...
Wanderer wrote in message . ..

It would have been nice to see an actual schematic of the network at
Hurst s/s. They have three supergrid t/frs on site. I'm guessing that
each would almost certainly be independently controlled, and
consequently capable of isolation, by circuit breakers on the hv and lv
sides of the t/fr.

Not necessarily so if this was configured as a mesh connected 4
breaker site or a three and a half breaker bay with the SGTs banked
with incoming feeders and disconnected by inter-tripping and power
operated dead break isolators.

Yes, a mesh site as is Wimbledon. I assume that only a few breakers
capable of interrupting live circuits are provided for reasons of
cost, if so this is surely a false economy that negates much of the
expensively provided diversity of transmission lines. The public and
political 'knee jerk' response is likely to call for more lines when
connecting all lines and SGTs via 'live breakers' would effectively
fire-wall most faults at a fraction of the cost.

Buchholz alarms are usually t/fr specific in the control room, so why
did the NG control engineer apparently disconect the incoming circuit
and not isolate the tranformer indicating the alarm? If this had
happened the overload situation would not have occured.

I wonder if the bucholz alarm was not for the main SGT, but for a
Voltage Transformer associated with the 275kV incoming circuit feeding
the bar.

The alarm was for a specific SGT and associated shunt reactor but
didn't show which (was later identified as the reactor which NG
isolated and re-energised the SGT at 23:00) but the only breakers to
isolate it also cover a transmission line which happened to be
carrying most of the S London load. The plan was to open the breakers
for a few mins while the SGT was isolated and then close them again,
the load displaced onto the line that tripped only took it to 33% of
rating - there was *no* overload anywhere.

The breakers were planned to be open for no more than 10 mins,
probably much less, during which it was accepted that the normal
standard of being immune to a double circuit (pylon) failure was not
met and it puzzles me why NG control didn't urgently complete the
planned switching thus reconnecting the unsupplied section within a
couple of minutes, the report doesn't explain this. Presumably they
re-energised in many small steps as a standard procedure to guard
against potential faults being worsened, because of possible damage to
the supplied system (EDF when offered supplies took up to ten minutes
to accept) and because it may be damaging to reconnect ~800MW in one
go. In the event I wonder if they had 'bitten the bullet' and
reconnected in 2 mins it wouldn't have been a much better outcome.

It would also be interesting to compare areas of responsibility and
manning levels for this part of their network now and say 25 years ago.
What level of authority and/or delegation of that authority holds today
compared to 25 years ago?

NGC's main control centre is at Wokingham in Berkshire, which I have
visited. They have two backup control centres, locations of which I
cannot reveal in public. The control of the network is split into
three stations across two control rooms, in direct communication with
each other with CCTV and Sound. The SCADA system is extremely
comprehensive and list almost every alarm condition and control every
circuit breaker and powered isolator on the system. 45 persons are on
shift at any one time. Previously, NGC had 7 or 8 control centres with
more limited facilities. Alarms were often grouped and the control
engineer often had to send staff to site to determine which alarm was
actually operated.

The report does say that alarm grouping and presntation requires
further work, the engineers had to deduce that the alarm came from the
shunt rather than the SGT.

The installation of a 1A relay on a 5A seondary circuit is undoubtedly
an error, but the relays are stamped with the Seondary current on the
front panel. The relays concerned at Wimbledon are digital and thus
the multipliers and IDMT curves are set by software, not plug bridges
or bolted links.

The report concedes that the relay rating could've been more clearly
stated on the documentation so the engineers probably did see it was a
1A relay but didn't spot that 5A had been specified. The multiplier
was correctly set for 5A, infuriatingly it could have been
recalculated to work correctly with a 1A relay!
[...]
fault. A primary injection test would have revealed the discrepancy
with the 1A relay fitted to a 5A secondary circuit. I have equipment
which can circulate 2000A at 2V, and NGT have even larger equipment.
This seems to be a hole in their test regime.

I find it surprising that in 2 years the loading can't have exceeded
23% of rating, would've thought they'd want to give it a good run now
and then to guard against possible 'rusty points' syndrome and to
confirm it does work. October would seem to be a good time when the
duplicate line can be switched back in instantly when this line
would've false tripped. Is there any way of fine tuning the load on
alternative circuits? Before the increased load the line was carrying
only 72MW of New Cross's 359MW load the rest coming what seems to be
'the long way round'. If it had been possible to gradually transfer
load the trip would only have lost part of Wimbledon including LT.

I think that we shouldn't forget that the reliability of the National
Grid has actually improved over the last 15 years, with fewer
equipment failures, losses of supplies and interruptions despite
increasing loads and the deregulation of the energy market.

They quote independent comparisons showing their record is much better
than many other countries. This does seem to be a case of not spending
a few thousand £ costing millions to their customers and the public. I
think NG Control were shown to be unprepared for dealing with a large
emergency, I don't really blame them but there's clearly a need for
modelling major and improbable faults to feed into simulator training.

One other observation. If Mr Game is a quality Guru, perhaps he should
offer his services to NGT?, he obviously knows better than those
within NGT who have a professional knowledge of the system and its
requirements, a skill which Mr Game's posts show complete lack of
knowledge of, even though I and others have tried to educate him, to
the point where he accuses the more knowlegeable of us of knowing
nothing (see his earlier posts), despite the fact that some of us work
in the industry or hand in glove with it - for example I run the
electrification design department of NR.

Everyone is of course entitled to their view on usenet, I've expressed
mine as above, those who peddle half baked theories with no knowledge
and can't be bother to listen to those in the know can of course
broadcast their views till the cows come home, just the rest of us may
choose to cease to listen to their diatribe...........

I wonder how Mr Game will respond if, or probably when, he's involved
in an embarrasing cock-up...