Preparing the grid for large-scale renewables.
Dwayne Stradford is a transmission planning engineer at SAIC Energy, Environment & Infrastructure LLC. Previously he worked at AEP in the transmission operations section.
As many wind generation developers have done in the past, solar generation developers are now seeking to connect large-scale renewable projects to utility transmission and sub-transmission systems. Renewable portfolio standards have created a market for the construction of merchant wind and solar generation and many state renewable programs require that these plants be interconnected to distribution and sub-transmission systems to receive renewable energy credits from the state. Solar inverters are now commercially available in 500 kW and 1,000 kW sizes, and photovoltaic panel prices have fallen in recent years—which makes bulk power grid connectivity, and the associated higher interconnection costs for these solar arrays, more feasible.
Wind turbines that produce up to 3 MW per turbine are now commonplace in the United States, with more than 40,000 MW installed in the country at the end of 2010, according to the American Wind Energy Association (AWEA). Additionally, there are many planned off-shore projects that are likely to use turbines at or greater than 5 MW per turbine. The transmission interconnection queues across the country have proposed future wind and solar projects ranging from 10 MW to more than 500 MW at a site with interconnection voltage generally aligned with the size of the project—the larger the project, the higher the interconnection voltage.
Analysis of potential reliability impacts of connecting large-scale wind farms and solar arrays to the transmission grid is relatively new to long-range transmission planners. The definition of large-scale, for the purposes of this article, are renewable projects exceeding 10 MW in total capacity and connected to the transmission system at voltages above 30 kv. In the past, smaller wind farms were built with Department of Energy funding to test the viability of successfully integrating wind generation into the grid. Likewise, solar arrays were previously installed behind the meter, posing little to no reliability concern. However, these smaller facilities still need to be properly metered and accounted for in order to reveal the true state of load growth on distribution circuits. Not doing so would pose a risk in skewing load forecasts, which could then lead to transmission providers not being adequately prepared to execute system upgrades to both transmission and distribution networks in a timely manner.
Transmission service providers need to contemplate the following concern: with larger solar arrays and wind farms being proposed to connect to transmission and sub-transmission systems, are utility companies sufficiently prepared to handle the challenge of integrating these large intermittent resources?
Power companies have to go beyond simply building an interconnection station for the renewable resources to inject their green energy onto the grid. It will require a function of refining, and for some companies, even redefining the generation interconnection requirements for new renewable projects. The days of selecting a generic voltage schedule and assigning the standard operational power factor limits for these new renewable resources are over. These schedules won’t sufficiently address future 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 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 deviation event.
In NYISO’s FERC order, two items stand out regarding the use of these LESRs. Point No. 7 of the NYISO order is important to note because NYISO is taking an active role to install flywheels around its territory with the understanding that the integration of wind power is here to stay. Item No. 20 of the NYISO order is important because, again, since there’s no legislative obligation being placed on renewable resources to provide any additional mitigation equipment, some ISOs have taken a proactive role to install these devices for better system control.
Similarly, the Midwest ISO has initiated a FERC process (Docket ER09-1126) to address significant modifications to its energy and operating reserve markets tariff, based on the impact of integrating stored energy resources onto the bulk-power system. [Editor’s Note: FERC ruled on MISO’s filing on Dec. 31, 2009, and May 10, 2010.]
The need for storage energy resources becomes more apparent considering the wind-related capacity emergency that occurred in Electric Reliability Council of Texas (ERCOT) in February 2008. It’s important to note that declining wind production played only a partial role in this event, as discussed in a report published by the National Renewable Electric Laboratory (NREL) (NREL/TP-500-43373, June 2008). However, with more new wind farms being sited and installed in Texas, now is a good time to evaluate the opportunity to not only strategically install flywheel technology, but also to integrate dynamic reactive resources, such as SVC or STATCON into the transmission infrastructure plan. Doing so will build in some accountability for the fluctuating voltage and frequency levels created by wind energy facilities on the deployment horizon.
Moving forward, the industry will have to decide whether the two major transmission reliability factors—dynamic voltage support and system frequency management—need to be resolved by renewable resources, or if they should become a cost of doing business for each transmission provider and reliability coordinator. Again, each transmission service provider will have to critically evaluate their generation interconnection requirements as they relate to integrating new renewable resources. These green energy resources will aid in supplying valuable power to the bulk electric system, but pose some challenges to transmission reliability. With the clock ticking on the retirement of key based-loaded generation across the country, the industry needs to resolve this challenge sooner than later.