WHAT IS A SMART INVERTER ANYWAY?
by Bruce Barney, Solar Oregon Board Member
Ever since the 1920’s, when isolated utility companies started joining their separate, islanded systems together, the utility grid has looked and worked pretty much the same – big, central, generating plants connected to all consumers through an interconnected grid of transmission and distribution lines. Sure, computers came along and modernized grid control, advances in some technologies made changes in the equipment used, but the fundamental concepts of big, central plants feeding all the electrical users through a grid stayed pretty much the same.
Fast forward to today’s grid: the utility grid will probably see more changes in how it works in the next 10 or 15 years than in the last 100! The main driver for this huge shift in how the grid works will undoubtedly be the growing number of electrical consumers who also become electrical generators. From a residential solar system to a municipal biogas plant to a cogeneration unit in a manufacturing plant, more and more distributed generation will be on the grid.
“So what?” You may ask. Well, as utility grids see more and more distributed generation, there will be more and more need to manage/control all that generation. Much of this generation will be what is known as ‘intermittent’ generation – generation that can come and go at any time – generation that the utility cannot count on to be there all the time – generation like solar PV systems that have generation valleys when a cloud bank passes over and generation spikes when cloudy skies change to sunny skies. These intermittent distributed generation sources turning up and down relatively randomly are very difficult for a utility to deal with. If you couple this effect with the current inverter performance standards (that force an inverter to disconnect at the first sign of a grid disturbance), there is a possibility that PV could really be detrimental to the performance of the electrical grid.
Enter the ‘SMART INVERTER’! These new inverters will be designed to help the grid to deal with intermittent generation. They will be able to help the grid stay stable and at the correct voltage and frequency rather than contribute to problems! Ahh, our problems are solved!, right? Well maybe and maybe not…..
To get to where we need to be to allow lots of distributed solar, we will need to use smart inverters. We will need to modify the existing inverter standards to allow them to stay connected to the grid during minor grid disturbances, to change their output to assist the grid in staying stable. They will eventually (for bigger systems) be communicating with the utility so they can be controlled remotely to help the grid. We will have inverters whose functions are ‘randomized’ so they don’t all disconnect from the grid at the exact same time, (which can cause grid instabilities).
How do we know this is coming? Because it’s already here – well, actually not here, but in Germany. They have so much installed PV in Germany that they are actually retrofitting existing inverters with smart inverter technology because it is needed to keep the grid healthy. Retrofits, as you can imagine, are expensive; so we are starting to see a push in the US to use smart inverters now, before we actually need them, in anticipation of the day in the future when we DO really need them. California utilities are already pushing for all new PV sites to use smart inverters!
How much will smart inverters cost? As usual, that depends. Many inverters already incorporate lots of ‘smart inverter’ capabilities, but the ‘smarts’ are effectively turned off for the US market, (as required by current standards/regulations). To change these inverters so they have ‘smarts’ is pretty easy and inexpensive. The added cost for this type of smart inverter is very low. To incorporate all the desired features of a really smart inverter may add $150 to the cost of a residential size inverter….that may seem like a lot, but it really will be required when we get to high penetration levels of solar PV on our grids.
Now for a bit of technical stuff for the engineering types: smart inverters will ‘randomize’ their time of separation from the grid in the presence of ‘out of tolerance’ voltage or frequency – this will mean that the drop of the solar generation will be a steep slope rather than a cliff if you were to graph it. I’m sure you can imagine that it will be easier for a utility to make up for this lost generation if it falls off gradually instead of all at once. Before we can really make this happen, the standards for how inverters work (mainly IEEE 1547) must be updated to allow it. This process has already started, but standards development is notoriously slow.
Eventually inverters will be connected to their utility’s control – so the utility can ask for the inverter to tailor its output voltage, frequency, or power factor to help provide whatever the utility needs most – voltage support or VAR support or frequency support.
So – the takeaway here is that smart inverters are coming, probably pretty soon. There will be a cost, but we shouldn’t complain; because that extra cost will enable more of us to utilize clean green solar energy!