IEEE Power & Energy Magazine - November/December 2015 - 34

When there is substantial common-mode tripping of the
distributed PV due to widespread voltage swing, the loss of the
generation causes a system separation and collapse.
for a relatively sophisticated representation of the impact of
embedded rooftop PV solar.
Getting the available infrastructure right is only half
the problem. When zero marginal cost wind and solar
power is available, economic operation of the system dictates that other generation with higher marginal production cost be displaced. Some displaced generation is taken
offline (decommitted), and some is turned back (dispatched
to a lower power schedule). The difference is important for
transient stability analysis, as a decommitted plant does not
contribute, either positively or negatively, to maintaining
transient stability, whereas a synchronized plant producing a
different amount of power will have an impact. The calculation of this complex economic optimization, which mimics
the operational behavior imposed by markets and regional
transmission organizations, is accomplished through production simulation. In this work, detailed analysis of production simulation results from earlier Western Wind and
Solar Integration studies were mined to determine decommitment and redispatch procedures for the balance of the
generation portfolio.
The resultant snapshots provide a good view of how a
future Western system, with enough wind and solar generation to meet 33% of the annual
(electrical) energy needs of the
western United States, would
look compared to consensus-based
planning estimates for the same
future time.
Wind
PV 0.0
Wind
2.5
The WECC base planning case
Others 8.4
CSP 0.0
PV 0.0
Others
14.6
for
the light-load scenario repreDG 0.0
CSP 0.0
12.3
sents a future in which current
DG 0.0
renewable portfolio standard targets are met. Consequently, a significant number of proposed wind
and solar projects are included.
PV 0.2
Wind
The high-mix case adds even
Wind
4.4
CSP 0.0
4.0
higher levels of wind and solar.
DG 0.0
PV 3.7
This case represents a snapshot
Others
in time-a windy, sunny morning
CSP 0.9
19.9
Others
24.9
in the spring. A third, "extreme"
DG 0.0
sensitivity was developed using
the same renewable plant ratings
Production/Dispatch in Gigawatts
and locations as the high-mix
case. For this extreme-sensitivity
case, the maximum coincident
total wind and solar production
figure 2. Wind and solar generation in the light spring base case.
snapshots to all other similar operating conditions. Big planning decisions are made based on just a few estimated initial
conditions. Thus, selection of meaningful snapshots is critical and requires a fair amount of art and experience.
When evaluating future scenarios, especially those in
which the available portfolio of generation will change substantively from the present, getting the cases right requires
considerable care. Assumptions about the available infrastructure-i.e., physical equipment, including power plants
(new ones added, old ones retired), and transmission (new
lines, upgraded equipment)-require informed insight.
In this study, base conditions including transmission system upgrades and plant retirements that Western Electricity Coordinating Council (WECC) stakeholders deemed
likely within the next ten years were used. The initial study
scenarios were from these WECC planning cases for light
spring and heavy summer load conditions. Then, tens of
thousands of MW of new wind and solar plants were added
to provide comparable cases with a "high-mix" of renewables. This high-mix case adds current technology wind and
solar plants at geographically (and economically) appropriate locations throughout the system. A special "composite
load model" was added to thousands of locations to allow

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