IEEE Power & Energy Magazine - May/June 2017 - 86

on, power flow begins to decrease. this effectively means that
control of the system has been lost. the grid has become unstable, and the phenomenon is referred to as instability. beyond
180°, power flow actually reverses.
When the angle difference between the two ends of a transmission line approach 180°, the electric power relays, which
are intended to protect against electrical faults (short circuits),
"see" a zero voltage on the line; the sine of 180° is zero. Since
they're intended to trip the line whenever a fault appears, they
quite correctly open the circuit breakers when zero voltage is
detected, and that line is removed from the system. What happens is that a kind of zero voltage node occurs on the system
along an arbitrary interface, a series of more-or-less parallel
transmission lines. Once all the lines between one electrical
area and another are opened, those areas are electrically separated. that's called a cascading outage and system separation,
and it generally happens in seconds.

Interregional Coordination
and Early Computer Simulations
of the Power System Network

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in the early 1960s, the Consolidated edison Company of new
York (Con ed) and the Allis-Chalmers Company jointly announced plans to build the largest generating unit in the world
at 1,000 mW. this new unit was to have a capacity more than
twice as high as the next largest unit on the system. its first
planned location was to be on the manhattan side of the east
river, just north of 14th Street. but subsurface geology made
that site somewhat problematic, and the location was moved to
an existing plant site across the river, ravenswood. its official
name became ravenswood 3, but the local press soon dubbed
it big Allis.
Con ed executives had not notified other utility companies in
the northeast of its plans until the contract with Allis Chalmers was signed and a public announcement made. in fact, not
even Con ed's own planning engineers had been notified. the
company had conducted neither steady-state power flow nor
dynamic studies of the proposed unit's impact on the rest of the
eastern interconnection. Once the announcement was made
and a press release issued, the immediate reaction of neighboring utilities was surprise and indignation. their concern (and
the concern of Con ed's own planners) was that the interconnected system would not be able to support the sudden contingency loss of a generator running at 1,000 mW. immediately,
plans for an extensive computer study, to be conducted by all
major utility companies in new York and the other mid-Atlantic states, were drawn up. this study showed that the sudden
loss of the unit at its maximum output would cause a technical phenomenon called transient instability, where the system
is subject to cascading transmission outages, separation of the
power system into two or more electrical islands, and ultimately
widespread blackouts.
the computer study showed that if the output of ravenswood 3 never exceeded 625 mW, its sudden loss would not
cause instability; breakup and collapse of a large portion of the

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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - May/June 2017