David Hansen wrote in message . ..
On Fri, 29 Aug 2003 21:22:02 +0100 someone who may be Chris Game
wrote this:-

Two large failures seven seconds apart is stretching coincidence a
bit don't you think?

As I understand it the first fault was in equipment and the second
in a cable. If speculating I would guess that the second fault was
due to an undetected incipient fault on the cable, which was unable
to cope with an increased load.

David, I'm not too sure that is the case. Hurst is equipped with two
400kV to 275kV supergrid transformers and feeds New Cross via two
cable circuits. Under normal operating practice, Wimbledon has three
incoming 275kV circuits (power being absorbed by Wimbledon) and two
outgoing circuits to New Cross (power being supplied from Wimbledon to
New Cross).

From NGC's press statement on Friday the 29th, they admit that the
first fault was on a transformer at Hurst. (Note: "a transformer", not
both units) The second fault was that which occured at Wimbledon which
NGC states as "stopping flows on a 275kV cable between Wimbledon and
New Cross" causing the blackout.

Taken at face value, that appears fine, but probe deeper. Why could
power not be maintained via the other SGT at Hurst and the other cable
circuit from Wimbledon to New Cross?

Here is my theory:

1) There was an outage of at least one circuit between Wimbledon/New
Cross/Hurst for maintenance or renewal at the time of the outage, thus
limiting the freedom to manouever around the first problem at Hurst.
My bet is that this was one of the Hurst to New Cross feeders. At the
same time, the other circuit, being fed from the other SGT, goes down
as a result of the transformer failure.

2) From NGC's website, the Wimbledon to New Cross Feeder 1 and 2 have
a circuit rating of 810 and 660MVA respectively. From a summer loading
perspective, I doubt that the load exceeded 700MVA (peak winter load
is predicted at 1051MVA) On this basis, I would expect even the
smaller cable to be able to withstand this load for more than forty
minutes.

3) There was a protection operation at Wimbledon (whether really
called for, or a maloperation) which either:

a) Caused "Stuck Breaker" protection to operate (for real or
maloperation), which tripped out the 275kV bar (Causing the whole
275kV bar to be lost and hence cuts the supply to New Cross and causes
the loss of the 132kV bar, causing the loss of supply to Lots Road BSP
- thus affecting the Tube)

OR

b) There was some other protection maloperation causing the 275kV bus
to be lost (such as bus zone with the bus being operated solid),
although this is much less likely than (a) above.

The reason why I would pick 3(a), is that NGC spokesmen were reported
as saying that all similar equipment around the country was being
checked. Given that protection at 275kV is always duplicated with
different manufacturer's relays and uses different settings at each
site, I feel that it would be very unlikely that a similar set of
relay combinations and settings can be found at other sites. I am
inclined to believe that an apparently stuck breaker is the cause
here. (Stephen Timms in his interview stated that a circuit breaker
was to blame, although that could mean a physical failure or a breaker
protection scheme.)

Well, I will have the facts at my fingertips upon my return to work
tomorrow.

Richard