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Integrating Renewables

Opportunity for advancement or exercise in futility?

Fortnightly Magazine - March 2010

represent the fast-acting switching controls for both real and reactive power flow across the grid. They provide operators with a near real-time ability to implement controlling actions in response to system challenges. However, to be effective, they will require greater visibility and transparency of grid status, also in near real-time.

Synchro-phasor technology provides time-synchronized, sub-second data applicable for wide area monitoring and allows the system operator to operate the power system closer to operating margin. SPMUs take the sampling window from six seconds to 60 times per second and provide a GPS time stamp for all measurements. Phasor data will drive a new generation of monitoring, operator decision support and, ultimately, fast real-time controls to improve grid performance.

Software systems also are advancing in ways that will help integrate renewable resources. Control centers will see a new slate of applications focused primarily on wide-area monitoring and power system visualization. Such visualization tools can allow operators to enhance system status knowledge and highlight interconnection status and priorities.

A third major factor involves operational changes. Increased integration of wind and other similar renewables-based generation also will result in the need for newer and more advanced operational methods and processes. Some techniques that merit consideration in large interconnected systems as found in Europe, United States and China include wind-only balancing areas, ACE diversity interchange and second-tier control centers.

Wind-only balancing areas create a virtual balancing authority across multiple control areas, allowing each control area to reduce its overall reserve requirements needed to support the appropriate amount of wind integration. This mechanism leverages geographic diversity both from the generation from renewables as well as load.

ACE diversity interchange (ADI) involves pooling individual area control errors (ACE) to take advantage of control error diversity— i.e., sign differences associated with the momentary generation and load imbalances of each control area. By pooling ACE, participants likely will be able to reduce control burden on individual control areas, unnecessary generator control movement, and sensitivity to resources with potentially volatile output such as wind, and allow reserve sharing across control areas.

Second tier control centers represent a further step in system operations. The continued evolution of operational methods and processes might result in a need to provide increased operator supervision to these activities. This stems from increased importance in understanding probabilistic elements of the grid, such as wind and load forecasts, and the availability of distributed smart resources. The system operator will need to collect the data and formulate an optimal dispatch method that coordinates with transmission dispatch. The attention deserved by such a task implies the need for additional control room support. Whether an additional desk is added to existing transmission control rooms or a second tier control center is established to support a group of transmission control centers is unknown. However, such coordination of assets likely will be necessary.

A fourth factor involves the role of demand management, which offers near-term potential for smart-grid implementations, with substantial benefits for managing peak loads and generation requirements. Demand management could supply valuable ancillary services to accommodate ramping rates for renewable