From Mundane to Exotic: Managing Oscillations in High-Inverter Power Systems

Modern power grids are enormous, incredibly complicated systems, full of huge, powerful, and sometimes restless machinery connected together through a massive and ever-changing network of power lines. The art of planning and operating safe and reliable power grids requires continuous attention to the system’s dynamic behavior. In short, all the parts of the grid must play nicely with each other.

A key operational objective for grid system operators is keeping the power system running smoothly and at an acceptable equilibrium. But customers, Mother Nature, and more nefarious forces are constantly disturbing that equilibrium. Disruptions could be things such as end-users changing their consumption, lightning strikes to power lines or other violent weather events, failing equipment, and an endless list of other troublesome phenomena. The power system must endure these disturbances then return to equilibrium quickly and safely.

It is the tendency of power systems to “bounce around,” or oscillate, and that’s the topic of this blog. Concern about some of these oscillations, especially as power systems transition to low-emission energy sources that use inverters, keeps a sizeable portion of the system operations community and other stakeholders awake at night. The Energy Systems Integration Group is launching a new report, Diagnosis and Mitigation of Observed Oscillations in IBR-Dominant Power Systems: A Practical Guide, designed for people tasked with assessing the cause of oscillations in their system and identifying mitigation options; consulting the guide is the first step that follows “I see an oscillation. What is it? What do I do about it?”

By using a few everyday examples, I’ll try to illustrate what oscillations are, what causes them, and why they must be mitigated to ensure grids can operate safely and reliably.

Oscillations are evolving so that they are dominated by control behavior.

Highway driving is a useful analogy: picture being in heavy traffic on a major road with your cruise control set at 60 miles per hour, and other vehicles doing the same. You are trying to maintain speed, stay at a safe distance from others, and stay in your lane. But, inevitably, there is a pothole. Today you are driving your old 1950s vintage jalopy that hasn’t had new shock absorbers since the Sputnik incident. Your car lurches vertically, and bobs through several swings before settling down again. These are oscillations. When they’re bad enough, they create a hazard. They put the car equipment at risk, make control (like staying in your lane) difficult, introduce a host of other unwanted concerns, and might even cause other cars to oscillate or crash. Substitute lightning hitting a power line for the pothole, and swings of voltage or frequency for the vertical swings of the car, and you’ve got the idea.

Driving in the modern world introduces some additional parallels. There are new opportunities for unwelcome behavior. Consider: how would self-driving functions respond to the pothole? Does the active lane assist operate correctly? In the future, will active suspension help or hurt? In short, in the old days things were mostly mechanical and passive. Just as with modern automobiles, today, we are evolving into a world where there are many autonomous or “intelligent” controls actively participating in the behavior of power systems. And they are everywhere, in solar panels, battery systems, wind turbines, car chargers, air conditioners. The list is nearly endless, and especially concentrated on devices with inverters.  Inverters with fast, agile, intelligent controls are everywhere because they greatly increase the functionality and value of the resources they enable, but the opportunities for unanticipated outcomes increase.

Active controls can aggravate system stability.

When controls act aggressively in response to stimulus, there is a risk they will overreact and cause unstable behavior. Another relatable example is the instability of a poor audio system in a meeting room. Everyone has experienced the combined effects of volume being too high, speakers in the wrong position, poor acoustics, and so on. Sound waves are oscillations, too. The growing screech, commonly called “feedback,” is more accurately called “positive feedback.” The noise grows without bound until something can’t handle it. This is a quintessential unstable control system.