How inverters can help solar grow and keep grids stable

SEIA reports 600,000 installed solar systems currently in the U.S. and expects that number to reach 3.3 million by 2020. While it’s great to see this green source of power taking off, there are challenges to that growth, especially at the utility level.

The electric power grid was designed to deliver power from the utility to the user while surplus power from customer-sited projects flows the other way—from the solar system to the utility. This is called backfeeding. The grid can handle backfeeding, but how much depends on the age and complexity of the grid equipment. As more solar projects come online, more power flows back to the utility than is needed, which raises concerns for grid operators tasked with matching the supply with the demand.

“As solar gains a larger percentage of our power generation portfolio, its production variability needs to be addressed,” said Ryan LeBlanc, senior application engineer for SMA America. “Historically, utility operators simply wanted solar to provide real power and disconnect from the grid if a narrow band of voltage and frequency were exceeded. Now it’s increasingly important for solar to provide reactive power and stay online to support the grid under a variety of conditions.”

One component to rule them all

There is already a component smart enough to help solar projects get along with the grid into the future: the inverter.

“Inverters act as the brain of the PV system,” LeBlanc said. “They still convert DC to AC, but they also enable monitoring, decision making and control functions. Because of this intelligence, it’s the component best suited to perform the additional, essential tasks required of modern PV systems.”

The inverter’s job is diverse and will include even more duties to support the smart grid of the future. In addition to controlling its own output, it will be asked to provide reactive power support for the grid, which will improve grid stability and efficiency. It will also interface with a utility operator’s SCADA system, have the ability to be controlled dynamically and remotely and provide diagnostic information to aid operations and maintenance crews in identifying and remedying system issues.

Are inverters up to the task?

Most inverter companies who have experience in Europe and parts of Asia already have a technical blueprint for addressing many utility concerns with grid management and control features, according to LeBlanc. “The majority of these technical solutions were developed between industry participants and grid regulators when Germany first began requiring advanced grid management functionality in 2008 for residential inverters down to 3 kW,” he said. “Since then, inverters of all sizes have become incredibly flexible with many capabilities often far beyond what is asked of them.”

These “smart” inverters can also now be paired with energy storage for a more comprehensive solution that helps stabilize the grid. “Though there are battery-based inverters with grid management functions such as peak shaving and ramp rate controls, the cost of storage remains a barrier for its widespread adoption.” Still, the benefits of energy storage systems to the grid­—such as reduced peak demand and decreased heavy shifting—makes finding a cost-effective way to incorporate this technology important. Like solar, storage technology is rapidly evolving and the market is expected to soon see smaller, more affordable commercially available solutions.

Soonwook Hong, power systems engineering manager at Solectria-A Yaskawa Company, explained that many inverter manufacturers can include grid management functions with minor software changes or firmware upgrades. Hong is supportive of inverter grid support functions, saying they are crucial to promote renewable energy usage. He also shares considerations about their effect on parties involved and on the inverter itself.

One example relates to when inverters are asked to limit generation. “The plant owner is compensated for their investment with real power generation,” Hong said. “Currently, there is no compensation model for the generation of reactive power. This has been discussed in the power industry for decades, not just the solar industry alone. Consortiums are discussing how to compensate the site owner for lost generation, but haven’t reached a resolution yet.”

Another consideration relates to inverters operating more hours a day or at higher loading conditions. “This can make their internal components run hotter and reduce their life,” Hong explained. “Inverter cost and reliability are dependent on the functions they provide.”

State standards and requirements

Europe, and Germany in particular, has solar penetration levels much higher than North America. At times, Germany produces more than 50% of its peak energy needs through solar, LeBlanc noted, yet it maintains one of the world’s most reliable grids. Europe has served as an excellent example for U.S. utilities to formulate policies. “Utilities consider system reliability paramount to providing reliable power,” he said. “Interconnection standards are developed to facilitate this mission.”

Hawaiian Electric Company (HECO) has been among the first utilities to set grid standards for solar in the United Sates. Hawaii’s high solar penetration level (12%), older grid technology and limited size originally proved problematic for HECO, which barred adding new solar installations for a short time. After orders from the state and further research into how updates to circuits and meters could allow the grid to handle more than first thought, the utility is working to approve its backlog of projects. It has set guidelines for grid interaction, including a published list of inverter equipment that will work on the grid.

California has had its share of issues because it currently has more grid-tied PV than any other state. To solve them, the California Public Utilities Commission (CPUC) is reviewing Rule 21, while includes interconnection, operation and metering regulations for connecting distributed generators (solar plants) to a utility’s electric system. The Smart Inverter Working Group (SIWG) is responsible for establishing inverter functions to comply with the rule. LeBlanc expects the standards developed by this group to likely be adopted by other states.

Hong said with 3,000 utility companies in the United States, it’s not easy to set or meet requirements, but it is a must to earn utility support of solar on the grid.

“Amidst the overwhelming benefits that solar power provides to our economy, environment, climate and energy mix, like all power generating sources, it also has challenges to overcome,” LeBlanc said. “Fortunately, we have a field-proven model for addressing these challenges based on lessons learned in other places.”

Some inverter grid-support functions

• Active power curtailment: The adjustment of a resource’s active power in various response timeframes to assist in balancing the generation and load, thereby improving power system reliability. When solar produces too much (as established by grid operators), the inverter increases PV voltage to reduce the power output of the array.
• Reactive power control: When voltage and current aren’t in-phase, you get reactive power that moves back and forth in the grid. This power can help grid operators regulate voltage on a timeframe of hours or days. Through SCADA systems, utilities can tell the inverter how much reactive power to let into the grid.
• Power factor control: Inverters can set the ratio of reactive power to active power, on a cycle to cycle of the AC line, to help maintain voltage.
• Voltage ride-through: Inverters can help maintain solar plant operation through periods of lower grid voltage to avoid having to disconnect, which causes a chain reaction of other plants disconnecting due to the dip in voltage, known as cascading. This helps keep the grid stable.
• Frequency ride-through: The inverter can help keep the solar plant from disconnecting from the grid during time of high or low variations in frequency, determined by regulatory requirements, therefore aiding grid stability.
• Ramp-rate controls: The inverter can control the rate at which it transitions between different established power factor points. This ensures the plant output does not ramp up or down faster than a specified limit. Energy storage technology can add or subtract power to or from the PV output to smooth out the high frequency components of the PV power.

More information on smart inverter grid functions is available from the Electric Power Research Institute at epri.com.