On 12 Sep 2003 11:50:36 -0700, Richard Catlow wrote:

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.

I think you've just proved my point. The report that has been published
is still woefully lacking in detail, leading to continued speculation.

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.

The report seems to be quite specific about the alarm and it's
associated equipment.

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,

snip

I obviously didn't choose my words carefully enough. I was speculating
on work on site within the substations, associated with the testing and
commissioning of equipment, rather than NGT's control procedures.

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.

Thank you for the clarification.

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.

Do you have evidence to support that statement? I don't question your
claim, but it would be nice to see supporting evidence. I will, however,
admit to a very healthy dose of engineer's cynicism when it comes to the
validity and reliability of such evidence. I spent too long in the
industry.