We Gotta Get Out of this Place
Today we are seeing massive build out of wind and solar PV generation in places that are remote from denser population centers that constitute the lion’s share of load. Around the world, decarbonization plans will accelerate this trend. Earlier this year, Chris Clack told you about the desirable, massive overbuild of these resources. The reality is, they won’t give the benefits we need if their power is bottled up in the hinterlands.
The problem of getting power across long AC transmission from remote areas with gigawatts of generation isn’t new to the industry. When I was a kid, the industry did “coal-by-wire,” and a quiver of technical and analytical solutions evolved to make sure the grid stayed stable. But when the power is coming from inverter-based resources (IBRs), we aren’t in Kansas anymore, Dorothy (well, maybe we are, it’s really windy).
The Nose of the Tiger
The dynamic behavior of IBRs from the instant the grid is perturbed out to a few seconds is fundamentally different. In this window of time, traditionally addressed by stability studies using phasor-based tools (like PSS/e, PSLF, TSAT…), stability is all about hanging on for the wild ride following an upset. Think: how big a pothole can you hit at what speed, and not end up in the ditch?
Those of us that worry about such arcana, understand that traditional transient stability limits surround the energy involved in a disturbance; specifically, we worry about dissipating the energy accumulated in synchronous machine rotors during faults. The familiar equal-area criteria gives a proxy for that entire class of behaviors. But with IBRs, the physics changes. The stability behavior of an IBR-dominant system is superior in many regards, but not all. The limits are a lot more about voltage stability than they are in a synchronous machine–dominant world. That means that we need to pay closer attention to the character of the grid — focusing on post-disturbance impedances, voltage support across the corridor, reactive regulator tuning, etc. — and less about the energy involved in the specifics of the disturbance. Consequently, fault duration is less important, and metrics like critical clearing time provide less information. Power-Voltage (PV) nose curves, and their extension to PQV solution surfaces (as shown, with the trajectory of a transient voltage collapse in red), give insight into whether the system has a safe place to land, and whether it will get there. New voltage stability metrics will need to emerge.
To be sure, not everything changes. We need to stay keenly aware of the power flows. IBRs don’t invalidate Kirchoff’s laws. The language “angular stability” somewhat displaces “rotor angle stability.” Slipping a pole is now a virtual event — the Zoom of the stability world.
If the Band You’re in Starts Playing a Different Tune
Of course, we are not charged with making a system that works just some of the time. Our systems will have mixes of synchronous and IBRs — not just generation but condensers, batteries, SVCs, etc. — that need to stay stable over ever-increasing daily and seasonal variation. That means the introduction of competing dynamics with, for example, synchronous condensers cohabiting with multiple massive wind plants. In new work sponsored by GridLab, Matt Richwine (Telos Energy) and I see the emergence of multiple, partially decoupled, stability phenomena. In this simulation, a marginally stable system with lots of IBR generation and stabilizing synchronous condensers exhibits two distinctly different phenomena. The voltage swings at about 1.6 Hz, driven primarily by the IBR regulators trying to settle to a stable equilibrium near the end of the post-fault PV nose. The power swings at about 1.0 Hz are driven primarily by the electro-mechanical oscillations of the condensers hunting for torque equilibrium. This syncopation makes understanding and mitigation more challenging. When we add grid-forming controls to the IBRs, some of this will get simpler, some not so much.
Climb Ev’ry Mountain
The industry is on a steep learning curve, but we are far from starting from scratch. The tools and understanding that are already at our disposal will take us far, as we learn better what to look for. But those tools are going to need to be augmented with new customized ones that give better insight as we shift away from electromechanically dominated stability limits. Stay tuned: this is going to be fun.
Nick Miller
HickoryLedge, LLC
Peter Jones says
Nick,
I would be very interested in following your blog. Sun Cable, http://www.suncable.sg, is undertaking a feasibility study for potentially the largest solar farm in the world (it’ll get beaten) in Elliot Northern Australia at 14GW and BESS for a firm 3.2GW transmission to Darwin as HVDC and exporting 2GW to Singapore a mere 3,800km subsea route. Darwin will take the rest.
My interest is in the solar farm branch circuit protection as we will be creating a macro grid of purely IBR as DC Coupled ~5MW blocks connected to primary 33kV AC OHTL onto 400kV AC that will feed each pole of the voltage source converter to HVDC for the 750kM run to Darwin. The traditional school of thought to have reactive grid protection circuit breakers by injecting reactive power into the network is being challenged by having predictive monitoring and proactive control over sections of the grid at the lowest common denominator/element possible and using the huge amount of data available coming all from the same source under same ambient conditions to analyse and provide steady state predictive maintenance control and fast fault clearing response. Would appreciate your thoughts and of course others on this.
Regards Peter Jones
Electrical Engineer
Sun Cable
Garth Gum Gee says
HI Nick
If you are interested keep abreast of the issues happening in Australia with a long stringy network built in the 1950’s and 1960’s to supply country towns now being required to host GW’s of IBR generators. It is almost a world laboratory. Not only trying to solve the physics but the market issues at the same time.
During 2019/20 there were hundred’s of GW’s of IBR constrained while the market operator could work out how to securely allow existing and new generators to connect.
Ken Wilson says
I have been following Nick’s work for several years and I have become increasingly concerned that utilities in the west are not researching these issues adequately. As an engineer who worked at Bell Labs in the 80’s and 90’s and helped the Bell System manage through big changes in technology, I have seen changes lead to big, unexpected problems. To remind everyone of an old engineering phrase: “shit happens”. Having moved from telecom to energy ten years ago, I was horrified at the lack of data available to the engineers managing the electric grid, in comparison to those who manage telecommunications networks. The information available has improved a bit in the past ten years, but is still woefully inadequate. And the risks are very high. As an advocate for renewable energy, my big fear is that we will have outages that are directly attributable to high, or even moderate penetrations of renewables in weak grid situations. As Nick reminds us in his presentations, we are getting close to thresholds in some areas of the country (and other countries) where there is enough penetration of inverter based renewables that grid stability issues can bite us in a big way. I hope enough attention is paid to these issues to avoid the potential problems.
Carlos Aviz says
Why do we assume there will still be synchronous machines? In Brazil, without Gas Turbines, our only synchronous machines would be hydro which will always run. Are the current positive sequence models adequate to represent IBRs for transmission studies? Do battery resources inherently have GFM capabilities and can these capabilities be enabled simply by adjusting control strategies? We should not relate all the instability problems to converter control, we should understand some instabilities imposed by the nature of weak grids. Is IEEE2800 anticipated to be applicable to international arenas, all system sizes, various strengths/weaknesses, etc.?