Can Synthetic Inertia from Wind Power Stabilize Grids?

p1110724As renewable power displaces more and more coal, gas, and nuclear generation, electricity grids are losing the conventional power plants whose rotating masses have traditionally helped smooth over glitches in grid voltage and frequency. One solution is to keep old generators spinning in sync with the grid, even as the steam and gas turbines that once drove them are mothballed. Another emerging option will get a hearing next week at the 15th International Workshop on Large-Scale Integration of Wind Power in Vienna: synthetic inertia.

Synthetic inertia is achieved by reprogramming power inverters attached to wind turbines so that they emulate the behavior of synchronized spinning masses.

Montréal-based Hydro-Québec TransÉnergie, which was the first grid operator to mandate this capability from wind farms, will be sharing some of its first data on how Québec’s grid is responding to disruptive events such as powerline and power plant outages. “We have had a couple of events quite recently and have been able to see how much the inertia from the wind power plants was working,” says Noël Aubut, professional engineer for transmission system planning at Hydro-Québec.

The short answer is good, but not good enough to support massive wind power growth. Québec has about 3,300-MW of wind power today, but Canada’s wind industry is calling for 8,000-megawatts more by 2025. Turbine manufacturers are upping their synthetic inertia technology to pave the way.

Synthetic inertia is the latest step in a longstanding technology trend, according to Aubut, that has already transformed renewable generators from potential liabilities to power grid stability into substantial contributors to it. The first step, he says, was equipping renewables to remain solid and thus “not harm the grid” during times of grid instability. Modern wind and solar plants are designed to “ride-through” severe faults, such as short-circuit events that drop grid voltage to zero.

Recent ride-through trouble in Australia appears to be an anomaly. Nine Australian wind farms did shut down during a series of storm-induced faults, that blacked-out the state of South Australia in September, and Australia’s prime minister attacked renewable energy as a threat to energy security. However, an investigation by the Australian Energy Market Operator blamed errant wind farm control settings, and it says some operators have corrected them.

In fact, most wind and solar farms can do much more than just stick around during trouble. For example, most utility-scale installations—and even some residential rooftop solar systems—are designed to combat voltage sags on power grids. Their electronic inverters can detect brownouts and generate reactive power (AC whose current wave leads its voltage wave) to raise the grid voltage.

Synthetic inertia is about responded to crashing AC frequency, usually after the loss of a big power plant. When a big generator goes offline, it leaves the grid under-supplied. That will cause the AC frequency to fall.

Conventional power plants respond naturally and instantly to frequency dips because the momentum of their spinning turbines, synched to the grid, resist deceleration. This slows the frequency drop, buying precious seconds during which power reserves are mobilized to fill the supply gap.

Aubut says Hydro-Québec began setting requirements for synthetic inertia in 2005. Québec’s grid is, electrically speaking, North America’s smallest AC zone, with peak power demand under 40,000 MW. Losing a big power plant causes a steeper frequency drop on smaller grids, and more wind power threatened to limit the Québec operator’s defenses.

In 2005 the utility amended its grid code, requiring wind farms to pull their weight: it mandated that new wind turbines be capable of delivering a power boost equal to 6 percent of their rated capacity during low-frequency events. Manufacturers responded with synthetic inertia designs, and the first were installed in 2011. Today, inertia-compliant turbines from Germany’s Senvion Wind Energy Solutions and ENERCON account for two-thirds of Quebec’s wind capacity.

To emulate the inertial behavior of massive rotating equipment, a renewable generator must somehow find extra power quick. Québec’s wind turbines do so through a collaboration between the turbines’ solid-state power electronics and their moving parts. “When the wind turbines see an imbalance between load and generation that causes a frequency deviation on the system they’re able to … extract some kinetic energy that is stored in the rotating masses of the wind turbines,” explains Aubut.

During a December 2015 transformer failure that took more than 1,600-MW of power generation offline, synthetic inertia kicked in 126 MW of extra power to arrest the resulting frequency drop. Quebec’s AC frequency bottomed out at 59.1 hertz – well below its 60-hertz standard – but Aubut and his colleagues estimate that it would have dropped a further 0.1-0.2-hz without the synthetic inertia. And they estimate that this was roughly the same contribution that conventional power plants would have provided.

“If we had had only synchronous generation instead of wind with the same event and operating conditions, we’d have had about the same deviation,” says Aubut.

The trouble, says Aubut, is what happens after the frequency drop. In all but the strongest wind conditions providing synthetic inertia will slow a wind turbine’s rotor. Re-accelerating to optimal speed thereafter absorbs some of the wind power that the turbine can export to the grid. Data from ENERCON shows power reductions of up to 60 percent in some turbines.

This energy recovery phase delays the grid’s frequency recovery. After Québec’s December 2015 transformer event, for example, the system frequency flat-lined for several seconds at 59.4 Hz before additional power reserves could push it back to 60. Under different conditions, says Aubut, that post-inertia recovery could have actually caused a “double-dip” in system frequency, increasing the risk of triggering protective relays at substations and causing blackouts.

Hydro-Québec is revising its synthetic inertia to minimize the risk of a double-dip. It plans to limit power reduction during recovery to no more than 20 percent of a wind turbine’s capacity. Turbine manufacturers are already testing second-generation synthetic inertia systems that comply with the new standard.

ENERCON presented an upgraded synthetic inertia control scheme at last year’s Wind Integration Workshop. Whereas the first generation of ENERCON Inertia Emulation revved rotors back to their optimal speed as quickly as possible, the new scheme uses power estimation and closed-loop control to enable smooth and tunable re-acceleration.

Markus Fischer, ENERCON’s Montreal-based regional manager for grid integration, says the upgraded scheme showed “promising results” in tests on full scale turbines and commercial rollout is “expected to happen in the near future.” Retrofitting its first generation machines, he says, will require no added hardware.

Synthetic inertia requirements, meanwhile, may be spreading. Grid operators in Ontario and Brazil have already joined Hydro-Québec’s lead, and Fischer says the first harmonized grid code for European generators, which entered into force earlier this year, “opens the doors to European system operators to ask for inertial response from wind.”

This post was created for Energywise, IEEE Spectrum’s blog about the future of energy, climate, and the smart grid

Wind Could Provide Over 26% of Chinese Electricity by 2030

Last month I argued that the primary reason Chinese wind farms underperform versus their U.S.-based counterparts is that China’s grid operators deliberately favor operation of coal-fired power plants. Such curtailment of wind power has both economic and technical roots, and it has raised serious questions about whether China can rely on an expanding role for wind energy. New research published today appears to put those concerns to rest, arguing that wind power in China should still grow dramatically.

The report today in the journal Nature Energy projects that wind energy could affordably meet over one-quarter of China’s projected 2030 electricity demand—up from just 3.3 percent of demand last year.

In fact the researchers, from MIT and Tsinghua University, project that modest improvements to the flexibility of China’s grid would enable wind power to grow a further 17 percent. That, they argue, means that China’s non-fossil resources could grow well beyond the 20 percent level that China pledged to achieve under the Paris Climate Agreement. Continue reading

Solar Power Towers Aren’t the Avian Annihilators Once Thought

Solar power towers have had a reputation as alleged avian vaporizers since preliminary reports emerged in 2014 of birds being burned in mid-air as they flew through the intense photonic flux at California’s Ivanpah solar thermal plant. Their reputation was muddied even more during tests early this year at SolarReserve’s Crescent Dunes power tower in Nevada; the solar thermal plant just recently began producing power. California public radio station KCET reported that as many as 150 birds were killed during one six-hour test in January.

It is obviously upsetting to imagine birds ignited in the name of renewable energy. (KCET reporter Chris Clarke, who has tracked the issue since BrightSource Energy began building Ivanpah in the Mojave Desert, described burning birds as “beyond the pale” in a recent article suggesting that power towers may be finished in California.)

But, upsetting as any killing of birds is, avian mortality is a downside common to many modern human creations—including buildings, highways, and powerlines. The best data on bird mortality at Ivanpah, macabre as it might be, shows the death rate to be small and likely of little ecological significance.

Meanwhile, operational adjustments at both Ivanpah and Crescent Dunes are pushing avian impacts even further below levels that could threaten local bird populations. “The data does support a low level of avian mortalities and hopefully, through adaptive management and deterrence, it will go even lower,” says Magdalena Rodriguez, a senior environmental scientist with the California Department of Fish and Wildlife. Continue reading

Arizona Utility Blinks in Bitter Battle Over Rooftop Solar

Arizona’s biggest utility, Arizona Public Service, is withdrawing its bid to jack up monthly fees for rooftop solar users in its territory. The retreat, tendered last week to the Arizona Corporation Commission (ACC), capped an eventful month in the high-stakes battle between utilities and solar advocates that’s raging across Arizona rooftops. The party with the most bruises is not Arizona Public Service (APS), however, but the ACC itself. The elected body referees the state’s power markets, but all five of its commissioners now face accusations of bias that challenge their ability to fairly adjudicate the rooftop solar dispute.

Arizona’s solar dispute is hot, but not unique. Across the United States utilities are fighting to contain or eliminate “net metering” policies Continue reading

Storing Solar Energy: A great idea caught on contested ground

Adding energy storage to sites with rooftop solar power generation offers a range of potential benefits. A battery can help smooth out solar’s inherently variable supply of power to the local grid, and even keep buildings powered during blackouts. Consequently, power-conversion innovators are developing a host of new products designed to reduce the cost and improve the efficiency of integrated solar-storage systems.

Some analysts project a boom in the co-location of solar and energy storage. GTM Research, for example, foresees that co-located PV and storage will grow from $42 million in 2014 to more than $1 billion by 2018. However, the market is moving slower than it might thanks to a little-discussed regulatory roadblock in the United States.

According to Vic Shao, CEO for the Santa Clara, California-based energy storage startup Green Charge Networks, tightly integrating storage with photovoltaics in some key states—including Hawaii and California—runs afoul of the “net metering” rules by which PV owners earn lucrative retail rates for the surplus power they feed to the grid. Adding storage can disqualify solar systems for net metering, in which utilities can pay their owners wholesale power rates that are several times lower than retail. “That is obviously a pretty big problem for anybody considering solar. That could kill a lot of projects,” says Shao. Continue reading

European Grid Operators 1, Solar Eclipse 0

Solar forecast for March 20 via Energy-Charts.de, with previous days' generation

Solar forecast for March 20 from Energy-Charts.de, with prior days’ solar output

Weather forecasts calling for bright sun today across Europe drove up tensions in advance of the partial solar eclipse that blocked the sun’s rays and plunged much of the continent into a brief period of darkness this morning. Grid operators were bracing for record swings in solar power generation because of the celestial phenomenon. Some power distributors in Germany had warned of fluctuations in frequency, notifying customers and suggesting that they shut down sensitive equipment.

In the end, while clear weather made for some excellent eclipse viewing, the electrical story ultimately felt more like Monty Python’s radio coverage of the 1972 eclipse. As if audio coverage of a quintessentially visual event isn’t absurd enough, the Pythons closed their fictitious report in the ultimate anticlimax, as a sudden rainstorm swept in to spoil the solar spectacle. Europe’s interconnected power grid brought about an equally anticlimactic ending today by delivering rock-solid stability throughout the 2.5-hour eclipse. Continue reading

Solar Eclipse Will Test European Power Grids

imgA partial eclipse of the sun headed for Europe next Friday has grid operators in a tizzy. On the morning of March 20 Europe’s skies will darken for the first time since solar power became a meaningful piece of some countries’ power supply, and the impact could be dramatic.

“It’s a very, very big challenge for the transmission system operators in Europe,” says Enrico Maria Carlini, Head of Electric System Engineering for National Dispatching at Rome-based Italian transmission system operator Terna.

The Brussels-based European Network for Transmission System Operators for Electricity (ENTSO-E) judges in an eclipse impact analysis released last month that it poses a, “serious challenge to the regulating capability of the interconnected power system.”

While an eclipse markedly reduced solar generation in western North America last October according to energy tracking firm Opower, Europe’s far greater levels of solar power make for bigger stakes. ENTSO-E projects that the moon’s jaunt across the sun’s path next Friday could slash more than 30 gigawatts (GW) of solar generation in Continental Europe over one hour if clouds are scarce and solar generation is high. That’s the equivalent of turning off 30 big coal or nuclear power stations. Continue reading