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Green Power Control

Preparing the grid for large-scale renewables.

Fortnightly Magazine - June 2011

operational control issues on the bulk power system. System operational control issues, such as system frequency control, will become very evident once the major legacy generation units are retired from service.

If renewable resources aren’t going to be obligated to contribute assistance for better voltage support or system frequency control, then transmission service providers will have to be prepared to provide adequate system resources for each renewable project that is coming online.

Critical Transmission Factors

Studying the intermittent nature of renewables adds complexity for the transmission planner in providing reliable service to electric customers. Modeling these large-scale solar arrays and simulating their volatile nature can be challenging, especially for real-time contingency analysis purposes. For example, cloud cover over a 30-MW solar array can cause fairly quick increases or decreases in the array output, potentially causing voltage regulation issues with load tap changers, regulators, and switched capacitors. Conceptually, these new renewable resources might require some type of flywheel technology (or comparable equipment) to be simultaneously installed in order to account for these sudden, and sometimes unpredictable, deviations in renewable power output.

From the drafting of interconnection agreements, in conjunction with the performance of interconnection studies, one burning question remains: have transmission providers been proficiently screening renewable resources thoroughly enough? In particular, there are two critical transmission factors that renewable resources provide significant challenges to the reliability and stability of the bulk power system.

The first critical transmission metric is voltage support, as it relates to dynamic reactive resource availability to the transmission system. Often, renewable resources are obligated to maintain as close to unity power factor as possible at the interconnection point. There is rarely any mention of a voltage schedule requirement for the renewable site during its online production of energy. In permitting this standard mode of operation, there has been a great tendency to place most, if not all, of the burden on the existing generation fleet to provide the adequate voltage support for its neighboring renewable resources. As it relates to best utility practices, if these new renewable resources had some type of dynamic reactive resource available at the interconnection substation, such as a static var compensator (SVC) or a static condenser (STATCON), this would begin to alleviate the stress placed on legacy base loaded generating facilities to support system voltages.

The second critical transmission factor is system frequency management. With the output volatility of these major renewable resources, there has been little to no accountability assigned to these renewable resources to ensure that adequate frequency mitigation equipment is in place. The lack of frequency control equipment has placed considerable stress on legacy generation facilities to compensate for the fluctuating power output of nearby renewable resources. This frequency mitigation could be in the form of the flywheel system, or other similar technologies.

In addition to these critical transmission operating factors, utilities have current generation resources scheduled to be retired within the next five to 10 years. These base-loaded resources have been making up for the voltage and frequency slop that has been created by the influx of various renewable resources