IEEE Power & Energy Magazine - November/December 2015 - 36

California-Oregon
Interface Power Flow (MW)

8,000

3

P (mm)

6,000
4,800 MW
4,000

1

2

2,000
0
-2,000
-4,000
-6,000

1
2
3

0

1

2

3

4 5 6
Time (s)

7

8

9

10

Heavy Summer Base
Heavy Summer Base with High COI Flows
Heavy Summer High Mix with High COI Flows

figure 5. Transient stability of the California-Oregon
Interface.

During heavy summer load conditions, the addition of high
levels of wind and solar generation increases the availability
of power in the Northwest that is economically attractive in
California. This tends to drive heavy loading of the major
north-south paths, the Pacific AC and DC Interties, to about
their present limits (path ratings). High power flows on the
California-Oregon Interface (COI) are known to be stressful
and to require a generation-tripping remedial action scheme
(RAS) for large disturbances such as blocking the power flow
of the Pacific DC Intertie (PDCI).
Three cases for simulation of a PDCI block are shown in
Figure 5 to examine the issue. In order to isolate dynamic
performance causality, these cases do not include any existing RAS the WECC has put in place to mitigate the known
stability problems created by this particular event.
For the base WECC planning case, shown in blue, the predisturbance flow on the COI is below the stability limit (path
rating) of 4,800 MW (noted by the horizontal line in the figure). The case is stable, and the post-disturbance flow settles
around the path rating. A test case, shown in green, has exactly
the same level of wind and solar generation as the base case,
but with additional hydro generation in the Pacific Northwest
dispatched to displace generation in California. This dispatch
results in the COI being loaded at the same limit as the highmix case. The system performance degrades significantly:
the system goes unstable and separates at about four seconds
(three seconds after the event). This is not, in itself, evidence
of a problem: if the RAS were enabled, the system would
be stable. The third case, using the high-mix condition with
much higher wind and solar generation, is shown in red. For
the high-mix case, the initial flow on the COI increases to the
path rating. (The economics of this particular condition would
36

ieee power & energy magazine

have resulted in even higher flows from north to south, if the
path ratings allowed it.) The transient stability for this case is
slightly better than the test case, with the separation occurring
after about 3.5 s.
This suggests that the transient stability of the system for
one of the well-known and critical events in the Western Interconnection is not fundamentally changed by high levels of
wind and solar generation. Heavy power transfer affects transient stability more than the different characteristics of wind
and solar. This conclusion is not a statement that the system
behaves identically. It is possible, and perhaps likely, that the
system dynamics are sufficiently different to require somewhat different levels of generator tripping or different arming criteria for the RAS. A complete evaluation of the current
practice to check for refinements would be prudent. There is,
however, nothing in this analysis to indicate that the system
dynamics have fundamentally changed and that radically different means to ensure stability for this event are required.

Coal Displacement and Weak Grid
The regional transmission system for the Northeast Region
of the Western Interconnection was designed based on the
size and location of the large coal power plants there, which
thus became critical nodes in the network. As a result, transmission system operators have historically counted on those
plants to provide the voltage and reactive power support
needed for reliable operation. Displacement of those central
plants by more dispersed wind and solar generation results
in those nodes being less well supported. With the regional
generation mix and plant locations altered, there is a risk of
local voltage and thermal problems occurring.
Mitigation of localized problems by conventional transmission reinforcements, such as reconductoring, adding
shunt compensation, or selective addition of circuit segments, is well understood. But a broader question arises as to
whether massive displacement of large central station thermal plants by more distributed wind and solar plants will
result in substantive changes in the regional and Interconnection-wide transient stability.
Figure 6 shows the change in dispatch with increasing
wind and solar production in the Desert Southwest and Northeast regions for the light spring base, high-mix, and extremesensitivity cases. Since this is a spring light-load condition,
most of the gas-fired generation is already decommitted, and
hydro is backed down to practical minima. (The gas group
includes some other thermal generation, such as cogen,
that is difficult to dispatch down or decommit.) That leaves
mainly coal generation to be displaced as the wind and solar
power comes in. Coal generation in the two regions is the
lowest color in the bars. In the two regions, the coal dispatch
is reduced by about 80% in the high-mix case. Most of the
coal plants in the regions are decommited, and the remainder
are dispatched downwards. In the extreme-sensitivity case,
the coal is reduced by about 90%. The high decommitment
of coal did not overstress the system, but local voltage and
november/december 2015



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