Network integration of wind power - Wind Power Integration: Connection and System Operational Aspects - page 107

conditions and n-1 is a single circuit outage. n-2 is an ambiguous term, with the
following possible meanings:
● n-d/c -System normal with the outage of a double-circuit tower line.
● n-1-1 or n-m-T -A circuit trip when another circuit is being maintained.
● true n-2 -Two unplanned and independent outages occurring simultaneously.
This later interpretation is an expensive standard, and whilst some networks
were originally built with it in mind, probabilistic assessment rarely justifies
maintenance of that standard as the network use grows. A corollary is that the
network is a much less expensive cost element than generation plus fuel, so
facilitation of the wholesale electricity market is strategic. In the end, whatever
the economics, environmental and stakeholder aspects associated with new
transmission circuits make it hard to achieve additional circuits for greater
security.
● n-g-1 -A network outage arising during an unplanned generation outage. This
could result in serious consequences. Since rotating machines are inherently
less reliable than lines and cables, an unplanned outage of a machine could
well occur during a line outage. This is tested by increasing output on all other
generators to match load. The extra output might violate constraints on parti-
cular generation nodes. To prevent this happening, utilities have developed
rules for generation security, e.g. above 1,200 MW of generating capacity there
should be four circuits on at least two routes. The above generation security
standard, while seemingly expensive, does offer the comfort to generation
project financiers that a new station has a very small chance of losing its route
to market, and thus helps to secure lower risk-premium funds.
There is a bridge between the purposes of the network in achieving load security
and generation connection security. If an unexpected outage of a generator can be
accommodated without loss of load, then the issue of generation connection security
is largely economic. In general, the unexpected loss of a generator is covered by
setting a minimum level of spinning reserve related to the largest unit infeed to the
system (the normal loss risk). The load being thereby secured, it then becomes a
matter of whether the funding agents for a generating plant will require a high level
of security in network connection. There is likely to be some relaxation in the
requirement for generating plants with low load factor (e.g. peaking plant). However,
this relaxation may not be allowed by funding agencies, because missing a few hours
of generation when the wholesale energy price is very high can affect the economics
of the generator. For completeness of explanation, the catastrophic failure of a bus-
bar coupler in a generation substation is very rare but could result in several gen-
erating units being lost simultaneously. This is the infrequent loss risk and is partly
managed by allowing the frequency to decrease to say 49.5 Hz for a period. Most
wind farms could never create a level of generation loss of this magnitude.
4.4.2 Wind farm connection issues
In the case of wind farms, with an expected load factor of 30-40%, it can be argued
that the above generation connection security thinking is an uneconomic luxury.