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.


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. In which case the control engineer would have no option but
to de-energise the circuit. Operational procedures for certain VT's
call for immediate switchouts of certain 132kV, 275kV and 400kV VT's
to prevent explosions. The SGTs at this site step down to 132kV and if
they failed they would not have caused an impact upon the 275kV
circuit. My money is on the VT as this would cause the loss of a 275kV
circuit and some outgoing 132kV circuits. NR lost both Bromley grid
132kV circuits as a result of this.

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.

With the demise of oil filled equipment and circuit breakers and the
extensive use of on line condition monitoring, the need for
maintenance staff is less than before, but more highly qualified on an
individual basis.

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. These are faster and much more relaible and are a
standard fitment on NR systems. At 275kV all protection systems are
duplicated with different maufacturers relays to ensure that a
malfunction with one set will not prevent a trip in the event of a
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 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.

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...........

Richard