Energy microgrids have emerged as more than just a curiosity. The technology is improving, costs are falling, and developers are lining up to build projects. How will microgrids overcome the...
Green Power Control
Preparing the grid for large-scale renewables.
being steadily added to the bulk-power system. Without industry intervention to develop new North American Electric Reliability Corp. (NERC) legislative policy addressing these issues, the bulk power system will face two significant future challenges: 1) potential voltage collapses and wide-area voltage depressions due to the lack of adequate dynamic reactive resources under certain contingency events; and 2) large frequency deviations due to the lack of appropriately installed energy storage devices to serve as system frequency stabilizers.
New renewable resources need to be cognizant of their lack of significant reactive contributions to the interconnected grid. These new renewable resources need to at least sustain their own voltage by achieving reactive resource self-sufficiency at the interconnect point to the transmission system. Most interconnection agreements have an arrangement for these renewable resources to be connected to the grid based upon a specific power factor or voltage range. These renewable sites are then obligated to maintain that contracted power factor or voltage range while operationally on-line supplying power to the grid.
Additionally, transmission service providers and transmission operators have established policies that sometimes will exempt these renewable sources—under a facility megawatt output threshold, typically anywhere from 10 MW to 50 MW—from maintaining a certain voltage or power factor schedule. Some utilities might explicitly exempt renewable resources under 50 MW from maintaining a voltage schedule due to their volatile nature. Most of these internal utility corporate policies are being driven by NERC reliability standards, namely VAR-001-1 (Requirements 3, 4, and 6.1) .
In the near future, it’s imperative for the industry to explore the drafting of NERC legislation in the VAR category to obligate these renewable resources to become more self-sufficient in sustaining their own voltage support at the interconnection point. With additional renewable resources coming on-line, getting industry buy-in to establish voltage support criteria at renewable sites larger than 1 MW and at voltage levels higher than 60 kv shouldn’t be an issue. Otherwise, the stage will be set to place an undue dynamic reactive burden on legacy generation resources. This legislation will have to be initiated through the standards development process at the NERC website or addressed through the various renewable resources operating groups at the regional entity (RE) level or reliability coordinator (RC) level.
Lately, some independent system operators (ISOs), such as New York Independent System Operator (NYISO) and Midwest ISO, have pushed to install flywheel technology within their operating footprints to address the fluctuating power output from renewable resources. NYISO has taken the frequency stability component very seriously and has addressed it in a tariff proposal. Federal Energy Regulatory Commission (FERC) Docket Orders ER09-836-000 and ER09-836-001, regarding NYISO’s petition to revise its services tariff, have indentified the importance of integrating flywheel technology into their transmission footprint in order to better accommodate the integration of wind generation farms (127 FERC para. 61,135, May 15, 2009) . This frequency compensation device has been identified as a limited energy storage resource (LESR) due to the maximum 15-minute period it requires to fully charge or dissipate its maximum energy in response to a system frequency