IEEE Power & Energy Magazine - November/December 2021 - 62

✔ The Interconnections Seam Study compared various
levels of increased connection between the EI and WI.
The HVdc macrogrid design [Figure 4(b)] saved money
compared to not building across the EI-WI seam. For
the 50% renewables case, the macrogrid scenario cost
US$18.9 billion more in transmission but saved US$3.5
billion in new generation capacity, US$35.7 billion
in operations and maintenance, and US$8.6 billion in
emission costs (compared to not building across the
seam). This yields a benefit-to-cost ratio of 2.5. At 95%
clean electricity (85% renewables), the benefit-to-cost
ratio was even better: 2.9 for a macrogrid compared to
not building the macrogrid. In the 95% clean electricity
scenario, the transfers across the EI to the WI seam
were roughly 40 GW, 30 times the current 1,300-MW
transfer capability.
✔ ZeroByFifty found that an HVdc network linking neighboring
states [shown in Figure 4(c)] would be near optimal
in terms of capital cost while removing some siting
hurdles. It also found that to reach 100% clean energy,
the total system costs by 2050 would be US$1 trillion
higher if the network is not built. ZeroByFifty's HVdc
capacities between the interconnections vary by scenario,
but the approximate transfer capacities are 38 GW
between the EI and WI, 30 GW between the EI and ERCOT,
and 8 GW between the WI and ERCOT.
✔ There are similar findings outside the United States.
The European TYNDP 2020 identified that by 2040,
an additional 93 GW of transmission infrastructure
would be needed on top of the 2025 grid status to
serve the national trends scenario with 75% of renewable
electricity. This would have an annual investment
cost of €3.4 billion but save €9.6 billion in
generation costs annually across Europe, at a benefitto-cost
ratio of 2.8. The European proposals for decarbonization
are more futuristic and ambitious than
America's. An HVdc/hydrogen grid is one example,
as described in " Transmission for Energy Islands in
the North Sea. "
Transmission Enhances Resource
Adequacy
Transmission does more than deliver resources to load: it
connects regions to provide a diversity of wind, solar, and
hydro resources as well as to provide load diversity. For
example, the RIIA found significant increases in wind and
solar effective load-carrying capability (ELCC) when those
resources were aggregated using MISO-wide transmission
rather than only serving local resource zones and customers.
The RIIA found, at the 100% renewables penetration
level, that MISO's wind and solar had an ELCC of 12.5%
if it served only MISO load, and a much higher ELCC of
24.6% if it served the combined loads of MISO, PJM, the
Southwest Power Pool (SPP), and the Southeastern Electric
Reliability Council because of the greater load diversity.
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Diversity benefits have been demonstrated in the SPP.
It reduced its planning reserve margin (PRM) from 17.6%
before 1998 (before the SPP's formation) to 13.6% during
1998-2016 to 12% in 2017. The expanded transmission network
and balancing area footprint that led to the reduced
PRM in 2017 was expected to save US$90 million annually
in deferred capacity investments.
A potential future resource mix that is very high in wind
and solar resources could potentially be vulnerable under
severe weather conditions, which create very high net loads
covering large regions over multiple days in a row. Large-scale,
interregional transmission (far exceeding the current levels)
will likely make the grid more resilient under such conditions.
Transmission Helps System Balancing
The RIIA differs from many other studies in that it steps through
10% increments of wind and solar penetration over time, conducts
a comprehensive evaluation of grid reliability, identifies
reliability violations, and applies mitigation options to those
violations before moving to the next increment. This approach
mimics real life, as opposed to snapshot studies that optimize
around a future end state, such as 100% clean electricity.
The RIIA used a production cost model to ensure the
hourly performance of each penetration level. The study also
examined changes in ancillary services requirements due to
increased renewables and the deliverability of those ancillary
services. The RIIA found that major transmission solutions
were not needed for energy adequacy at lower (G 30%)
renewables' penetrations because an overbuild of wind and
solar capacity [the purple bars in Figure 5(a)] was sufficient.
However, at higher penetrations, additional transmission
expansion [the gray areas show ac, orange shows dc additions
in Figure 5(a)] was required to provide diversity in renewables
and load.
Transmission Helps Steady-State Reliability
The RIIA's load flow analyses examined thermal and voltage
violations under steady-state conditions for normal and abnormal
operations. The operating violations started occurring at
30% renewable penetration, requiring transmission expansion
[Figure 5(b)]. Additional higher-voltage transmission solutions
were required as renewable penetration increased.
Transmission Needed for Dynamic Stability
The RIIA's analyses addressed frequency response, transient
stability, small-signal stability, and converter stability
to examine dynamic stability in the event of a transmission
fault or the loss of the largest generator. At 40% renewable
penetration, the pockets of the system with high penetrations
of inverter-based resources experienced weak grid issues.
These resulted in undamped voltage oscillations and control
interactions [Figure 5(c)]. The RIIA first applied low-cost,
commercial solutions, such as controls tuning and generator
redispatch, but as voltage and converter stability issues
became more severe, more expensive solutions were required.
november/december 2021
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