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You're welcome ... here's a bit about the trees and why trees present(ed) a problem shown as in an inset in the US-Canada Joint Report on the blackout:

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Why Did So Many Tree-to-Line Contacts Happen on August 14?

Tree-to-line contacts and resulting transmission
outages are not unusual in the summer across
much of North America. The phenomenon
occurs because of a combination of events occurring
particularly in late summer:

- Most tree growth occurs during the spring and
summer months, so the later in the summer
the taller the tree and the greater its potential
to contact a nearby transmission line.

- As temperatures increase, customers use more
air conditioning and load levels increase.
Higher load levels increase flows on the transmission
system, causing greater demands for
both active power (MW) and reactive power
(MVAr). Higher flow on a transmission line
causes the line to heat up, and the hot line sags
lower because the hot conductor metal
expands. Most emergency line ratings are set
to limit conductors? internal temperatures to
no more than 100 degrees Celsius (212 degrees

- As temperatures increase, ambient air temperatures
provide less cooling for loaded transmission
lines.

- Wind flows cool transmission lines by increasing
the airflow of moving air across the line.
On August 14 wind speeds at the Ohio
Akron-Fulton airport averaged 5 knots at
around 14:00 EDT, but by 15:00 EDT wind
speeds had fallen to 2 knots (the wind speed
commonly assumed in conductor design) or
lower. With lower winds, the lines sagged further
and closer to any tree limbs near the lines.

This combination of events on August 14 across
much of Ohio and Indiana caused transmission
lines to heat and sag. If a tree had grown into a
power line?s designed clearance area, then a
tree/line contact was more likely, though not
inevitable. An outage on one line would increase
power flows on related lines, causing them to be
loaded higher, heat further, and sag lower.

OF interest to a number of people that day, including me, was why did the blackout/blackout boundry stop where it did - they give some answers in this part of the report - there is more detail in the report but this is a good start:

Why the Blackout Stopped Where It Did

Extreme system conditions can damage equipment
in several ways, from melting aluminum
conductors (excessive currents) to breaking turbine
blades on a generator (frequency excursions).

The power system is designed to ensure that
if conditions on the grid (excessive or inadequate
voltage, apparent impedance or frequency)
threaten the safe operation of the transmission
lines, transformers, or power plants, the threatened
equipment automatically separates from the
network to protect itself from physical damage.
Relays are the devices that effect this protection.

Generators are usually the most expensive units
on an electrical system, so system protection
schemes are designed to drop a power plant off the
system as a self-protective measure if grid conditions
become unacceptable. When unstable power
swings develop between a group of generators that
are losing synchronization (matching frequency)
with the rest of the system, the only way to stop
the oscillations is to stop the flows entirely by separating
all interconnections or ties between the
unstable generators and the remainder of the system.
The most common way to protect generators
from power oscillations is for the transmission
system to detect the power swings and trip at the
locations detecting the swings-ideally before the
swing reaches and harms the generator.

On August 14, the cascade became a race between
the power surges and the relays. The lines that
tripped first were generally the longer lines,
because the relay settings required to protect these
lines use a longer apparent impedance tripping
zone, which a power swing enters sooner, in comparison
to the shorter apparent impedance zone
targets set on shorter, networked lines. On August
14, relays on long lines such as the Homer
City-Watercure and the Homer City-Stolle Road
345-kV lines in Pennsylvania, that are not highly
integrated into the electrical network, tripped
quickly and split the grid between the sections
that blacked out and those that recovered without
further propagating the cascade. This same phenomenon
was seen in the Pacific Northwest blackouts
of 1996, when long lines tripped before more
networked, electrically supported lines.

Transmission line voltage divided by its current
flow is called "apparent impedance." Standard
transmission line protective relays continuously
measure apparent impedance. When apparent
impedance drops within the line's protective relay
set-points for a given period of time, the relays trip
the line. The vast majority of trip operations on
lines along the blackout boundaries between PJM
and New York (for instance) show high-speed
relay targets, which indicate that massive power
surges caused each line to trip. To the relays, this
massive power surge altered the voltages and currents
enough that they appeared to be faults. This
power surge was caused by power flowing to those
areas that were generation-deficient. These flows
occurred purely because of the physics of power
flows, with no regard to whether the power flow
had been scheduled, because power flows from
areas with excess generation into areas that are
generation-deficient.

Relative voltage levels across the northeast
affected which areas blacked out and which areas
stayed on-line. Within the Midwest, there were
relatively low reserves of reactive power, so as
voltage levels declined many generators in the
affected area were operating at maximum reactive
power output before the blackout. This left the
system little slack to deal with the low voltage conditions
by ramping up more generators to higher
reactive power output levels, so there was little
room to absorb any system "bumps" in voltage or
frequency. In contrast, in the northeast-particularly
PJM, New York, and ISO-New England-operators
were anticipating high power demands
on the afternoon of August 14, and had already set
up the system to maintain higher voltage levels
and therefore had more reactive reserves on-line
in anticipation of later afternoon needs. Thus,
when the voltage and frequency swings began,
these systems had reactive power already or
readily available to help buffer their areas against
a voltage collapse without widespread generation
trips.