INTRODUCTION:
Do you remember that outage that left some 50 million people in the dark on August 14, 2003? It took down 61,900 megawatts of load in eight eastern U.S. states and the Canadian Province of Ontario. The financial impact was as high as $10 billion in the U.S. and $2.3 billion up north. When government researchers from the U.S. and Canada examined the event, they reported that insufficient reactive power was one of the factors leading to it.
So, here’s the big question: When rooftop solar installations start causing localized voltage headaches for utilities, will there be enough local reactive power to bump that voltage up? There will if we get smart inverters along with new solar deployments.
What’s so smart about that inverter?
Smart inverters do more than convert the direct current produced by a solar panel or stored in a chemical battery to the alternating current that runs on the grid and powers your washing machine. As the name implies, smart inverters have sensors and computing power built in. They also have communications technology that enables bi-directional flow of information and control.
But, here’s the important part: Smart inverters can help shore up frequency and voltage abnormalities that are headed our way once we have lots of distributed energy resources such as solar and wind power on our grids.
How? The inverters know what the voltage and frequency are at their terminals. So, if an overabundance of solar power starts impacting frequency, smart inverters can be programmed to dynamically step back the power being put on the grid. Likewise, they can synthesize reactive power, which means they’re able to boost or depress voltage, whichever is needed.
How? Reactive power maintains voltage at the ANSI standard levels necessary for each section of the grid, and it’s on the grid whenever current and voltage aren’t in phase. If the current waveform leads the voltage waveform, you have a leading power factor. The reverse of this – where current lags behind voltage – delivers a lagging power factor. Adding leading-current reactive power bumps up the power line’s voltage. Injection of lagging-current reactive power basically brings reactive power down, which also brings voltage down.
Fault ride-through is another important function smart inverters can deliver. Right now, most inverters in the U.S. comply with IEEE 1547 standards, which have conservative settings at which a device must trip off in the event of power-quality abnormalities. Fault ride-through can support system reliability in a couple of ways. First, if you have 10 percent or more of the energy on a feeder supplied by solar and all the solar on a line trips off simultaneously in response to out-of-tolerance voltage or frequency, the precipitous drop in power will exacerbate voltage problems. Smart inverters can avoid that problem. They also can be programed to trip off randomly, so the loss of power is a gradual event, not a sudden one.
In addition, voltage ride-through can mitigate the impact of an outage. Suppose you have a substation with five feeders coming out of it and a fault hits one feeder. That could bring down the voltage on the whole substation. But, if there’s a fault on feeder number five, the substation will detect it and trip off that one feeder. The voltage may go down for a little while, but it will pop back up again after feeder five trips off. Ride-through makes sure that all the solar generators on feeders one through four will continue to provide power.
Why we need smart inverters
The forecasted growth curves of household storage and small-scale battery energy storage are jaw-dropping.
Navigant Research predicted early in 2015 that distributed energy storage will grow from a $450 million industry now to $16.5 billion in 2024.
The solar industry supplied 40 percent of all new electric generating capacity installed in the first half of 2015, more than any other energy technology, notes SEIA. Solar installations grew by more than 30 percent in 2014, they’ll top 30 percent growth in 2015, and they’ll likely account for roughly half of new capacity installed annually through 2020, according to GTM Research.
That’s a lot of inverters soon to be coming on line! Now is the time to make sure they’re smart ones.
CONCLUSION:
Germany learned about the need for smart inverters the hard way. Once the country’s generation mix included some 30 percent of intermittent renewable resources, frequency and voltage problems became so widespread, regulators mandated smart inverters for new and existing system. The price tag for retrofitting systems sized between 30 kW and 100 kW was estimated to reach $300 million. Here in North America, we have the chance to avoid that mistake in the first place.
For more information on why we need smart inverters and smart software controls, read this Enbala white paperEnbala white paper.