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Sun, Solar and the Grid

Situated in the nation’s sunniest state, Arizona Public Service (APS) is taking a lead in evaluating the implications of high solar penetration in the power grid. In one experimental project, APS is installing solar production meters, in addition to regular smart meters, in residences with photovoltaic installations. Data taken every 15 minutes will be correlated with historical information about weather and irradiation, to produce solar production forecasts for individual arrays, neighborhoods and whole regions.

As a resident of Arizona, I enjoy many natural benefits of living in the U.S. Southwest: picturesque mountains, an abundance of outdoor recreational activities and beautiful weather. With more than 300 days of sunshine each year, my fellow Arizonians and I enjoy the benefits of the sun no matter what our personal interests or hobbies might be.

Harvesting the sun for solar power is one benefit customers of Arizona Public Service (APS) are beginning to capitalize on. To date, more than 20,000 APS customers have installed solar on their homes or business. These systems range in size from 2 kW to 20 MW, depending on customer type and location. As the cost of solar continues to decline, the number of customer applications for solar interconnections continues to rapidly increase with no signs of slowing down.

APS also continues to invest in its utility-scale renewable portfolio. By the end of this year, APS will have more than 750 MW of solar in its generation mix, enough to power more than 185,000 homes.

So with all the sun, systems and solar energy, what’s the problem?

With increasing levels of distributed solar energy being fed into the grid, utilities must plan for intermittent generation to be interconnected at potentially tens of thousands of points, at locations they cannot specifically control; depending on the time of day or year, power flows may be reversed. For example, solar generally produces power between 6 am and 6 pm, with peak production between 10 am and 2 pm. During the summer, APS’s distribution feeder peaks generally occur between 5pm – 9pm, Monday-Friday. Most customers consume the power they generate during the summer months due to the high air conditioner loads. But during the spring and fall months when temperatures and customer loads are down, solar production is at its highest. What will happen to power flows along the distribution feeder then? Utilities will need to make sure relaying, voltage regulation, VAR support, and overall power quality are maintained, while still delivering power and operating the grid in a safe and reliable manner.

Energy storage also has a role to play; the key will be to determine when commercial and economic viability has arrived. If the current price trajectory of lithium-ion continues and PV penetration continues to increase, battery storage will figure in the APS grid modernization plan.

APS has embarked on a number of strategic initiatives designed to improve our overall ability to incorporate high penetrations of renewable energy.

One of these initiatives is the Flagstaff Community Power Project, in part funded by the Department of Energy, in which APS is studying the effects of increasing levels of penetration of solar on the distribution system. Through the use of advanced metering, modeling and sensors, APS will identify system characteristics and mitigation techniques to help shape the future of advanced distribution feeder design and operations.

A second initiative includes the deployment of solar production meters on all customer-sited photovoltaic systems. Along with a bi-directional AMI meter used for monthly billing purposes (that is to say, a standard “smart meter”), a second AMI meter will be placed on the output of the solar system in order for the total production of the system to be monitored on a 15-minute interval basis, which a typical smart meter does not do on residential installations.

By monitoring individual system production, enhanced solar forecasting also can be accomplished. These forecasts can then be aggregated by feeder, substation or region depending on internal business needs. The 15-minute data are correlated with historical information about weather and solar irradiance. Then, by means of an algorithm and third-party weather services, hourly forecasts can be created for each PV installation. Depending on the business need, individual system forecasts can be aggregated by feeder, substation, regional area, or total system.

Output from the forecasting tool will be utilized by the utility's individual departments, such as system operations, distribution planning, rates, finance, and marketing and trading.

In addition to measuring production and usage, APS is working towards retrieving hourly minimum and maximum voltage reads from all of its AMI meters to help monitor voltage delivery levels. This will help ensure that regardless of time of day, year or location on the feeder, voltage levels are not impacted by local intermittent generation resources.

The overall APS strategic grid modernization plan is its third initiative . With an operational focus on grid situational awareness through “visualization and automation,” the plan includes a balance of technologies including the deployment of an advanced distribution management system, automated switches, substation health monitors, communicating fault indicators, integrated volt /VAR control, network protectors, and communication backhaul on the distribution system. On the transmission side, system improvements include additional phasor measurement units (PMUs), advanced visualization tools and enhanced real-time contingency analysis.

Through the use of these advanced operational platforms, robust system health monitors, and remote automation, APS will ensure that its grid is flexible and adaptable to meet the future needs of its customers, while delivering safe, reliable and cost-effective power. APS, its customers and the sun can help Arizona meet its goal of becoming the solar capital of the United States.


  • Tony J. TewelisTony J. Tewelis joined Arizona Public Service (APS) in February of 2008 and currently is Director, Technology Innovation. He is responsible for the overall strategic development and implementation of the company’s advanced technology. Previously he was responsible for the corporation’s renewable distributed energy program.

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About the Smart Grid Newsletter

A monthly publication, the IEEE Smart Grid Newsletter features practical and timely technical information and forward-looking commentary on smart grid developments and deployments around the world. Designed to foster greater understanding and collaboration between diverse stakeholders, the newsletter brings together experts, thought-leaders, and decision-makers to exchange information and discuss issues affecting the evolution of the smart grid.


Doug HousemanDoug Houseman, an IEEE member, is the Vice Chairman of the Intelligent Grid Coordinating Committee.
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Vadim ZheglovVadim Zheglov, an IEEE member, is a Power Systems Consultant at EnerNex.
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John D. McDonaldJohn D. McDonald is an IEEE Fellow, an IEEE Smart Grid technical expert, and a past president of the IEEE Power & Energy Society.
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Tony J. TewelisTony J. Tewelis joined Arizona Public Service (APS) in February of 2008 and currently is Director, Technology Innovation.
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Barbara LockwoodBarbara Lockwood is the General Manager of Energy Innovation for Arizona Public Service (APS).
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