One of the major hurdles to overcome in
harmonising and combining separately operating grids is making sure that the
systems are compatible and that the necessary fail-safes are in place to make
sure that there is not a cascading catastrophic failure of the grid. All
components of the grid must operate in unison and to within set frequency
parameters.
Short, localised outages occur on power
systems frequently. System wide disturbances that affect many customers across
a broad geographic area are rare, but they occur more frequently than a normal
distribution of probabilities would predict. Electric power systems are robust
and are capable of withstanding one or two contingency events, but they are
fragile with respect to multiple contingency events unless the systems are
readjusted between contingencies. With the shrinking margin in the current
transmission system, it is likely to be more vulnerable to cascading outages
than it was in the past, unless effective countermeasures are taken.
A cascade is a dynamic phenomenon that
cannot be stopped by human intervention once started. It occurs when there is a
sequential tripping of numerous transmission lines and generators in a widening
geographic area. A cascade can be triggered by just a few initiating events, as
was seen on August 14th. Power swings and voltage fluctuations caused by these
initial events can cause other lines to detect high currents and low voltages
that appear to be faults, even if faults do not actually exist on those other
lines. Generators are tripped off during a cascade to protect them from severe
power and voltage swings. Protective relay systems work well to protect lines
and generators from damage and to isolate them from the system under normal and
abnormal system conditions
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