Wanderer wrote in message . ..
On Wed, 10 Sep 2003 15:21:57 +0100, Roland Perry wrote:

In message , Wanderer
writes
Finally! Confirmation that the two faults were indeed related. As some
of us insisted (against solid opposition) from the start.

Not quite true. You conveniently snipped the *real* reason, which was
almost certainly human error.

The *reason* for the second outage might have been human error [1], but
the second outage was most definitely *caused* (ie triggered) by the
first. It wasn't an "unrelated" incident.

On any interconnected system, the National Grid being a prime example,
events are by definition related. This incident was caused by a
classic very simple installation 'cock-up' made 2 years ago and not
discovered or exposed since; system design, configuration and incident
response were correct and supplies would have been securely maintained
otherwise.

The second outage (Wimbledon - New Cross 275kV circuit No.2) which
caused the loss of supply was a 'phantom' overload trip, there was no
equipment fault and total load was 33% of line rating. Overload
detection relays, connected via 1,200:1 transformer, are rated 1Amp or
5Amp with a wide ranging multiplier, 5A was specified but 1A installed
and through all the checks nobody spotted the error. This meant that
tripping was initiated (time delay involved, irrelevant here) at 23%
of rating and 20% of correct overload setting, so in the past 2 years
this line can't have loaded above that level even at peak winter
demand indicating the generous level of redunancy provided.

It's accepted that when the system is being reconfigured, as it was
after the first, potentially explosive, fault warning; there can be 5
to 10 mins 'switching time' when supply is via a single circuit. The
risk of a second fault on that circuit during this time is estimated
at 1:40,000 though that doesn't apply here with the system wrongly
set-up. The engineers planning the switching could not have known the
line would trip.

[1] Although you could argue that it was really caused by failure to
implement a procedure that would have noticed and rectified that human
error.

Yes. Knowing the very strict and thorough commisioning and testing
procedures that once were enforced, one is left wondering whether
corners have been cut since privatisation.

They were carried out but I guess those engineers assumed they had
been handed over correctly installed equipment and never thought to
check the relay rating, pity they aren't brightly colour coded.
Simulating real life faults is simply not possible or wise.

Reading the full report my impression is of a well run and financed
operation not obviously damaged by privatisation with London area
investment running at £50m pa. Ironically the duplicate circuit,
Wimbledon - New Cross No.1, is out from 1/7 to 28/9 for upgrade and
refurb.

Lessons have been, painfully, learnt and the relay error is not likely
to happen again. All 45,000 are being checked (none found after
9,000). I would say the system is now secure, building extra lines
would be very expensive. The LT loss of supply is rather laid on EDF,
the London distribution operator, who chose to operate their Wimbledon
132kV Substation in a way that resulted in LT and others losing supply
when the Grid was able to maintain the full load without interuption,
and should have reconnected LT in a couple of mins at most. I get the
impression there was only one engineer in their Control who didn't
know what hit him...

Source:
http://image.guardian.co.uk/sys-file...on28082003.pdf