The Future of Advanced Inverters

Distribution-connected PV inverters with advanced functionality, also known as “smart inverters”, have become mainstream in recent years.  Analyses and field experience have demonstrated that smart inverters are a cost-effective alternative to achieve higher penetration of PV in distribution circuits and at the system level.  However, the full potential of smart inverters is yet to be realized.  This will happen over time, as the fleet grows, and as their capabilities are adopted more widely and put to good use.

Interconnection rules requiring grid support functions were first adopted in California (Rule 21), to address the effects of rapid PV deployment.  These requirements, being rolled out in a phased approach, effectively defined what we now refer to as smart inverters.  The CA changes quickly led to revisions to IEEE Standard 1547, approved in 2018.  The new IEEE standard requires grid support capabilities that were discouraged in previous version of the standard.   This is a significant development because IEEE 1547 is our national standard for interconnection of distributed energy resources (DER).  It paves the way for smart inverters to be deployed with PV and other up-and-coming waves of DER such as distributed storage.   PG&E estimates that, by 2028, all behind-the-meter PV in California will have smart inverters—that represents more than a few GWs!

Today’s smart inverters can do more than their predecessors.  Besides delivering energy to the grid as efficiently as possible, they are capable of actively regulating voltage, responding to frequency, riding through abnormal voltage and frequency, and changing operation in response to instructions delivered via communications.

Communication capability enables collections of smart inverters to be aggregated and managed as virtual power plants (VPP) by a third party or by a utility through a distributed energy resource management system (DERMS).  This enables DERs to be dispatchable and capable of delivering energy and grid support services at a scale comparable to large generators.  We know how to coordinate the operation of inverters inside a wind or solar power plant.  Pulling this trick with different types and sizes of inverters operating under retail tariffs is a whole different ball game.

Manufacturers are shipping smart inverters for new installations; however, some of the goodies are disabled out of the box.  California and Hawaii have adopted default settings for Volt-VAR and Volt-Watt functions, and most jurisdictions have or are considering default settings for ride-through.   Other functions such as DER disconnect/reconnect, limiting maximum active power, and setting active power have not been fully rolled out.  They will require some form of communications back to a supervisory control.  Storage and grid-forming controls will enable an even wider matrix of values to system owners and to the grid, including dispatch and the ability to form microgrids during grid outages.

Smart inverter functions are being introduced slowly. Aside from California and Hawaii, we have not agreed on default settings for the most basic grid support functions, nor have we figured out how to compensate for the value that full DER dispatch functions can provide to the grid.  Business models and regulatory implementation are lagging.  Control architectures, communication protocols and security for smart inverters are also a work in progress.  Meanwhile, gigawatts of smart inverters will be deployed with advanced capabilities built in, but disabled.  There is no other way to do it; ensuring that key functionality is ready to go when needed is well worth the incremental cost.

More on communications

In the mid-2000’s, under a DOE-funded project called Solar Energy Grid Integration Systems (SEGIS), Sandia worked with manufactures to develop first-generation smart inverters with advanced functions and communication-based controls.  This project was in full swing when I joined Sandia some 10 years ago.  Coming from a utility background, I had trouble with the notion of adding communication dependency and complex control schemes on distribution systems.  Such schemes are hard enough to implement on transmission systems, where situational awareness and communications is much better compared to distribution systems.  The cost of connecting DERs used to be a major factor, but things are changing radically at the edge of the grid.  Our insatiable appetite for faster-better-cheaper communications means that smart inverters can be plugged into the internet with little incremental cost.  This is enabling fleet monitoring and management, and delivery of performance data to customers in real-time.  Inverter manufacturers are developing platforms to make smart inverters and other DER part of the Internet-of-Things.  While it is technically possible to use the internet for VPPs or DERMS, security is a concern.  It is likely that control applications involving fleets of DER will leverage dedicated networks over cellular systems.

Looking to the Future

We have work to do to ensure that smart inverters achieve their full potential.  Technical and institutional issues need to be overcome before smart inverters are used to provide essential reliability functions, including voltage and frequency regulation.  Some of this will necessitate supplementing autonomous controls of individual smart inverters with supervisory controls of inverter fleets, via VPPs or DERMS.  We need to establish business models, market structures and regulatory frameworks that take full advantage of the capabilities of smart inverters.  As we progress along those lines, we will be able to squeeze even more value from aggregations of smart inverters; better situational awareness, more options for system recovery and restoration, adaptive protection and control schemes, etc.

Much of the future opportunity related to smart inverters will depend on the level of deployment.  The rollout of distributed storage will certainly involve smart inverters, but if we play things right, we will also have smart converters on other devices such as EV chargers and smart appliances.  We should move forward with full awareness of what is a stake. Today, smart inverters are a cost-effective way to increase DER hosting capacity.  Soon enough, though, we will be asking these little critters to help support the stability of the whole grid.

Abraham Ellis
Sandia National Laboratories