IEEE Power & Energy Magazine - November/December 2016 - 22

(a)

(b)

figure 1. The Chateauguay station: (a) the interior of the valve hall and (b) the exterior of the valve hall, harmonic filters,
and the switchyard. (Courtesy of Hydro-Quebec.)

on the Intertie would usually be south from the greater
hydroelectric resources in the northwest to the southwest
that relied on higher cost fossil generation. However, in
1994 the Pacific Northwest was in a drought, and hydroelectric generation was limited due to low reservoir and
river flow conditions. When the earthquake occurred,
physical damage to the Sylmar converter station (southern terminal) caused the HVdc facility to trip. (See Figure 2.) The northwest area was now generation deficient,
and automatic underfrequency load shedding operated
to arrest the frequency decline and allow the system to
recover. At the same time, many generators, transmission lines, and local loads tripped in southern California
due to the earthquake damage. As the frequency began
to recover toward 60 Hz, some additional generation in
the northwest tripped; power swings resulting from the
initial disturbance caused the Western Interconnection
to separate into several islands, and some additional
under-frequency load shedding occurred in Washington
and Oregon.
In December, while the Western systems were reviewing the investigation and implementing recommendations, a severe winter storm in the Rockies caused the
loss of major transmission lines into Idaho from Montana
and Wyoming that were transferring power into Washington and Oregon. As in the January event, large power
swings caused the Western Interconnection to break up
into several islands; in some of those islands, particularly the Pacific northwest, the resulting generation deficiency and frequency decline initiated load-shedding. As
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the frequency recovered toward 60 Hz, several generators tripped, causing an additional frequency decline and
another round of load-shedding. In terms of load and generation lost, the December 1994 event was significantly
more severe than the January event.
The investigation of the December event and a comparison of the two events revealed several failures common to
both events. Among these were slow governor response to
the frequency decline and the unexpected operation of several remedial action schemes and generation rejection. The
insufficient governor response was due, in part, to the limited availability of hydroelectric generation. With the Pacific
Intertie operating on a low power schedule or flowing north
and the systems in the northwest net importing, generation
rejection was not desirable.

New Meaning to the Term "Cold Load Pick-Up"
On the same day as the Northridge earthquake occurred,
a severe winter storm moved from the Midwest toward
the northeast. This was a very strong storm system with
high winds, heavy snow, and severe cold; the cold reached
nearly to the Louisiana, Mississippi, and Alabama coast of
the Gulf of Mexico, where temperatures dropped to near
0 °C. Over the next days, many generators were forced
to shut down due to fuel-handling problems, coolingwater intakes freezing, or pneumatic controls freezing.
Some utilities found it necessary to implement rotational
load-shedding due to insufficient generation to meet the
demand. Previously, utilities only had rare occasion to use
this form of load-shedding during summer peak conditions.
november/december 2016

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - November/December 2016