Accommodating High Levels of Variable Generation - NERC

Transmission Planning & Resource Adequacy
such inter-area transmission development to provide access to and sharing of flexible resources.
Therefore, the composite capacity value of variable generation resources significantly improves
when inter-area transmission additions allow variable generators across much wider geographic
areas to interact with one another, hence improving overall system reliability.
As such, the resource adequacy planning process should no longer solely be a function of
planning the resource mix alone. Transmission system expansion is also vital to unlock the
capacity available from variable generation. Further, in those regions with a competitive
generation marketplace, regulatory targets such as Renewable Portfolio Standards heavily
influence the location and timing of renewable generation investments and their development.
Furthermore, government policy and any associated cost allocations (i.e. who pays for
transmission, additional ancillary services and ramping capability) will be a key driver for
variable generation capacity expansion. Therefore, an iterative approach between transmission
and generating resource planning is required to cost-effectively and reliably integrate all
resources.
In summary, transmission expansion, including greater connectivity between balancing areas,
and coordination on a broader regional basis, is a tool which can aggregate variable generators
leading to the reduction of overall variability. Sufficient transmission capacity serves to blend
and smooth the output of individual variable and conventional generation plants across a broader
geographical region. Large balancing areas or participation in wider-area balancing management
may be needed to enable high levels of variable resources. As long as existing transmission
pathways are not congested, transmission expansion may not be required to achieve the benefits
of larger balancing areas or sharing ramping capability and ancillary services between adjacent
areas, depending on how existing and planned inter-area transmission assets are used.
Currently, high-voltage transmission overlay expansions are being considered in various parts of
the NERC footprint. High-Voltage Alternating Current (HVAC), High-Voltage Direct Current
(HVDC) transmission or a hybrid combination of both provides expansion alternatives for this
overlay approach. HVAC can flexibly interconnect to the existing AC grid, including tapping by
generation and load centers, as the grid evolves. However, for very long, over ground distances
(wind sites are hundreds of miles away from demand centers), or for special synchronous
purposes, dedicated HVDC may be a more suitable solution. In addition to long distances,
offshore applications also offer technical challenges that can preclude HVAC cables. With the
advent of voltage-source converter (VSC) technologies, additional HVDC benefits (e.g. reactive
power control voltage and frequency control) have proven useful for offshore wind plants 60 and
may be useful in other applications.
60 www.abb.com and www.siemens.com.
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