Advanced, Configurable Inverter Functions Help Harmonize Solar's Impact on the Electrical Grid
More people are realizing the importance of solar energy as a tool to diversify our energy portfolio and reduce our carbon footprint.
By Michael Mills-Price
More people are realizing the importance of solar energy as a tool to diversify our energy portfolio and reduce our carbon footprint. Growing acceptance of solar as a reliable and important technology, along with drops in module prices and Balance of System (BoS) costs, has led to unprecedented 2011 and 1H 2012 photovoltaics (PV) growth. In the U.S., solar makes up some 1 percent of our energy generation, but as PV penetration is expected to increase, new challenges arise. Although manageable, these challenges require the industry's attention to ensure long-term success of the PV market.
One challenge associated with the increase of PV interconnection to the electric utility grid stems from the design of the existing infrastructure, its protective control features and electromechanical equipment settings. The electrical infrastructure was traditionally designed to carry electricity from a central generation station out to customer loads. With the increase in distributed generation PV projects, rooftops, carports, brownfields and other areas are now becoming sources of electrical power, distributed throughout the electrical network and requiring a revisit to operation and control practices by the local utilities.
A second challenge comes from the intermittency and unpredictable nature of solar as an energy resource and the resulting grid variations and resultant impacts on the existing voltage regulation equipment under high penetration scenarios. With too much power, local voltage and its respective frequency can rise and cause problems for motors, lights and many other electronics. With too little power, voltage and frequency can sag. It's not a problem exclusive to solar, but worth noting because other energy sources-gas, nuclear and coal-are more controlled and predictable because they aren't dependent on weather conditions.
These two challenges take a variety of forms dependent on geographical location, utility control practices, local loading (circuit loading), and seasonal load and generation variations. Advancements in many user configurable inverter functions can overcome these challenges by providing a deterministic response at the point of interconnect, based on the need of the broader electrical system.
Inverters are widely recognized as the respective hub of a PV system, so they need to maintain a high level of user configurability to meet the needs of local engineering procurement construction services (EPC), customers and utilities.
In terms of the challenges previously addressed to widespread PV integration into existing electrical distribution circuits, technologies have been developed, including:
• Local scheduling capability,
• Voltage support functionality,
• Closed loop point-of-interconnect control, and
• Phasor Measurement Unit- (PMU-) based island detection.
These functions have been demonstrated and investigated under the Department of Energy's (DOE's) Solar Energy Grid Integration Systems Advanced Concepts (SEGIS-AC) program. Inverter manufacturer Advanced Energy's SEGIS-AC program leverages a systems-based approach to mitigate the voltage concerns caused by the intermittent nature of solar. This systems-based approach looks beyond the point of interconnect to address feeder wide power quality and voltage stability concerns, improving inter-operation and intra-operation of geographically distributed resources.
As power electronics based devices, inverters contain the necessary functionality to mitigate and suppress many of the voltage and frequency issues anticipated to emerge as PV becomes a larger percentage of the energy demands on distribution circuits. The key is reliability: The coordination and aggregation of these capabilities allows system operators to maintain reliable, efficient control under widely varying intermittency events that sometimes can be caused by PV systems.
Improvements in overall electrical system efficiency can be realized as real and reactive power can be generated close to point loads, reducing the losses associated with long distance power transmission. With power electronics based devices, the ratio of real and reactive power can be controlled in a linear manner, eliminating step changes in voltage often associated with electromechanical voltage regulation equipment-and resulting in a smoother voltage profile at the customers load points.
In terms of the challenge associated with distributed generation, further developments, again as part of the SEGIS-AC program, leveraging wide-area information for optimizing aggregate-geographically and electrically distributed-PV resource response are being demonstrated to highlight the values to the broader electrical system. This system state information, combined with the advanced inverter control functionality, enables optimization of the PV resource behavior at the distribution feeder level and above. This aggregation of the distributed resources allows virtual power plants to be coordinated and controlled in harmony with the individual circuit needs even if the PV resources are spread over a geographically wide service territory.
This extended utility system awareness, combined with advanced inverter functionality, allows more efficient operation of the electrical distribution circuits- enabling tighter control bounds and increased capacity for the subtransmission and transmission systems. With extended intelligence associated with wide area information, the inverter and PV system can act in harmony with the legacy electromechanical voltage regulation equipment-resulting in extended life, improved customer voltage profiles and lower system losses. Lastly, with increased awareness and visibility throughout the electrical distribution network, and combined with the capabilities to react quickly, these redefined inverters are allowing for vastly larger PV penetration rates on electrical systems without electrical circuit redesign and the associated cost implications.
PV growth in the U.S. and its penetration rates at the distribution circuit level are a crucial step to reduce our dependence on traditional power plant fuel sources and lower our collective carbon footprint. It does not, however, come without challenges, and Advanced Energy is at the forefront of creating technologies and demonstrating capabilities that allow this PV resource to comprise a larger portion of our nation's energy needs. With a variety of system needs-at the installation locations-Advanced Energy's approach has been to develop a platform of user configurable control and optimization strategies to allow seamless integration while maximizing the benefits to the greater electrical power system. Through advanced inverter and system control features, aggregation across service territories (virtual power plants) and deterministic response (priority driven schedulable control) have resulted in improvements to power quality, voltage stability and overall system efficiency in electrical distribution systems.
About the author: Michael Mill-Price is SEGIS program manager and technical lead for the Solar Energy business unit at Advanced Energy Industries. An original PV Powered employee and one of the principle designers responsible bringing the PV Powered product lines to market, he continues to be a forward looking design engineer and now leads a team developing technologies to enable high PV penetration into the utility electric grids. Prior to PV Powered, Mills-Price designed motor controllers for large traction vehicle drives (Hybrid electric vehicles). For more information, please visit www.advanced-energy.com or email Michael.Mills-Price@aei.com.