Adding renewable energy to power grid requires flexibility

April 1, 2015 by Anne Ju

Solar panels, wind turbines, electric vehicles and other green power sources are proliferating rapidly, but their reliable integration into the existing electric grid is another story.

A study led by Eilyan Bitar, assistant professor of electrical and computer engineering, offers a comprehensive reimagining of the that involves the coordinated integration of small-scale distributed . The study, commissioned by the Power Systems Engineering Research Center (PSERC), asserts that the proliferation of renewable energy must happen at the periphery of the grid, which will enable the local generation of power that can be coordinated with flexible demand.

Bitar's study outlines a new architecture to enable what he calls a grid with an intelligent periphery – a version of the so-called – along with coordination strategies and mathematical models to simulate how such a reorganized grid would work.

"The uncoordinated proliferation of distributed energy resources will wreak havoc at scale," Bitar said. "Certain components of the legacy power system will fail; the existing distribution infrastructure isn't equipped to accommodate, for instance, a large number of plugging into the grid at the same time under the same transformer … but, imagine taking all these new resources and coordinating their control."

The way the power grid works now, large plants deliver power to substations, where electricity is provided on demand to homes. The fluctuating electric loads at each substation interact over a complex transmission network, but nearly everything that happens below the substation level is left uncoordinated. Electricity is an example of a commodity with inelastic demand – in the U.S., people are used to having it all the time, whenever they want. Bitar thinks that mindset will need to change slightly.

All the talk of the smart grid wouldn't be nearly so complex if solar and wind, for example, were a reliable supply. But those resources are variable, and the gaps must be compensated by traditional bulk power generation, effectively defeating the purpose of a renewable source.

In an intelligent grid, this variability in supply would be balanced through the coordination of flexible distributed energy resources at the periphery of the system. Power would be produced locally and consumed locally, giving rise to self-sufficient communities or cities, called microgrids. Such an approach would decrease the need to transmit bulk power hundreds of miles to counterbalance fluctuations in renewable sources.

The architecture of such a system, which requires sensors and actuators in appliances, electric vehicles and the like, isn't the hard part, Bitar said. The hard part is the design of algorithms to efficiently manage the deluge of information produced by those sensors in order to coordinate the simultaneous control of millions of distributed energy resources on fast time scales.

Much of this coordination will involve using flexibility in demand to compensate for variability in supply. For example, electric vehicles, considered a viable alternative to internal combustion engines, need to plug into the in order to charge. But because they're battery-based, the demand for their charge is flexible – for example, utilities could offer monetary incentives to electric vehicle owners willing to shift their charging patterns. Coordinating that flexibility with a variable resource like rooftop solar could lead to increased penetration of this resource while supplying clean power to electric vehicles.

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gkam
3.7 / 5 (3) Apr 02, 2015
We will need a very advanced and secure system for integration into the grid. A lot of work is needed. But it will be the cost of progress.

How do we allow access? How do we maintain voltage level and frequency stability? Who decides who gets to generate? At what price?
gkam
3 / 5 (2) Apr 02, 2015
Want something to worry about? Look into the quality of the voltage waveshape put out by inverters in alternative energy generation. Electronic inverters only put out a facsimile of the sine wave, often in steps or in PWM. These are ripe with harmonics, which can turn to heat in equipment, screw up crossovers used for time accounting (in rare instances), and affect components in user equipment miles away.

Who provides the VARs needed for grid operation? (look it up) Who traps the harmonics?

Check into Power Quality.

gkam
3 / 5 (2) Apr 02, 2015
Power Quality is the silent threat to grids nationwide. The little electronic power supply in your computer draws pulses of current, high in third harmonics, which can overload the three-phase Neutrals with overcurrents where they were supposed to cancel, resulting in sometimes thrilling conversions of copper into vapor.

ASDs usually produce currents high in fifths, which act to actually try to turn motors backwards, but turning into heat in the windings.

I started working in customer facilities in the mid-1980's to solve some of these problems for PG&E, and it became the Power Quality Group after I left.

Eikka
not rated yet Apr 02, 2015
In an intelligent grid, this variability in supply would be balanced through the coordination of flexible distributed energy resources at the periphery of the system. Power would be produced locally and consumed locally, giving rise to self-sufficient communities or cities, called microgrids. Such an approach would decrease the need to transmit bulk power hundreds of miles to counterbalance fluctuations in renewable sources.


The variability problem of solar and wind gets worse on the small local scale, and less so on the large continental scale. In other words, microgrids need more storage capacity or alternative dispatchable energy sources in proportion to generation capacity.

The question is, which is cheaper, 30,000 miles of HVDC cable, or 3,000,000 MWh of batteries?

Which one is practically possible to build?
Eikka
not rated yet Apr 02, 2015
Power Quality is the silent threat to grids nationwide.


Hence why there are regulations that demand active PFC and load shaping out of devices such as computers. The standards have been tightened since the 80's.

http://power-topi...-it.html

Since 2001, the European Union (EU) established limits on harmonic currents that can appear on the mains (AC line) of switchmode power supplies. Today, the most important regulation is the "European Norm" EN61000-3-2. This regulation applies to power supplies with input power of 75 watts or greater, and that pull up to 16 amps off the mains. It sets severe limits on the harmonic currents up to the 39th, when measured at the input of switchmode power supplies.
gkam
3 / 5 (2) Apr 02, 2015
In other words, microgrids need more storage capacity or alternative dispatchable energy sources in proportion to generation capacity.
------------------------------------

Are you assuming alternative energy is restricted to wind and PV? Are you assuming it will not be practical to use alternative energy and storage?

Eikka
not rated yet Apr 02, 2015

Are you assuming alternative energy is restricted to wind and PV?


Not restricted per se, but wind/pv are the most scalable sources worldwide. The rest are either not universally available to make the generalization to all locations, or aren't able to scale up to meet grid-scale demand.

For example, burning biomass is limited to the availability of arable land not already used for producing food, which is already 40% of the earth's land surface, so it's not plausible to scale up bioenergy because of ecological reasons. We can't burn our forests to save the climate.

Are you assuming it will not be practical to use alternative energy and storage?


I don't understand the question.
gkam
1 / 5 (1) Apr 02, 2015
"Hence why there are regulations that demand active PFC and load shaping out of devices such as computers. The standards have been tightened since the 90's."
----------------------------------

Eikka,thanks for de-fusing one of the arguments against alternative energy, which for the most part does not use rotating generation!!
Eikka
not rated yet Apr 02, 2015

Eikka,thanks for de-fusing one of the arguments against alternative energy, which for the most part does not use rotating generation!!


I didn't.

First of all, the standard applies to electrical loads, not generators. Secondly, it's never been a problem to produce a sine wave. The issue is that rotating stock offers some amount of power regulation due to the fact that every spinning generator and steam turbine is in reality a huge flywheel; inverters are not, and do not.
gkam
3 / 5 (2) Apr 02, 2015
Okay, give me a clean one from your efficient inverter.

Yes, anything which creates current harmonics generate voltage harmonics through system impedances. That is why loads are regulated. If you think voltage inputs to the grid are not, keep thinking.

Harmonics in distribution systems turn to heat, overload transformers, and cause relatively high-frequency characteristics for a system optimized for sixty Hertz.
Eikka
not rated yet Apr 02, 2015
Okay, give me a clean one from your efficient inverter.


One of what?

Consumer grade inverters are rarely anywhere close to pure sine, but they don't need to because they aren't connected to the grid. They're used for running isolated systems off of batteries.

Grid tie inverters on the other hand use high frequency PWM up to hundreds of kHz, which means they can approximate a sine wave very well, and are efficient and easy to filter. They have greater switching energy demand though, so they don't become very efficient until the output exceeds a couple kilowatts.

The actual issue with these systems is that they have no energy storage, so they can't take part in frequency regulation of the grid. They either push whatever input power into the grid, or they disconnect entirely, instantly.
Eikka
not rated yet Apr 02, 2015
Another recent modulation technology is multilevel PWM that further reduces the harmonic distortion over simple high-frequency PWM. Example:

http://www.academ..._CURRENT

The total harmonic distortion can be pushed below 2.7% and put straight into the grid without filtering transformers in between, although in the US the regulations are lagging behind for allowing transformerless inverters on the grid.
gkam
1 / 5 (1) Apr 02, 2015
Thanks, Eikka.

I had gotten behind.
Mike_Massen
3 / 5 (2) Apr 03, 2015
PWM is well recognised method for producing the power curve necessary, select an interval & drive the duty cycle from low to high in that interval according to sine ratios, pretty easy to do with a micro-processor &a table, adding corrections for feedback & harmonics summing is routine...

Have a Fiskars 98% efficient transformerless inverter 60KVA & battery cabinet, even back in early 2000 these had low THD but meant for use in an enclosed room as the 6KHz whistle could be annoying.
http://niche.iine..._System/

In 1998 for the village power system at Mendulong, I ran PWM off 24vDC into the fields of (AVR) 3-phase alternators which went straight into a 3-phase rectifier for charging a 20ft container of lead acid batteries as the typical Remote Area Power System (RAPS) for the Sabah rural electrification program.
http://members.ii...s/Power/

AVR's used 68HC705C8's
http://members.ii...s/Power/avralt.jpg
Mike_Massen
1 / 5 (1) Apr 04, 2015
Solar to Hydrogen claimed
solar to Gas Unit provide Distribution at low cost to fuel cells and engines on Mini pipe line grids at low cost and low line loss when compared to the wold copper grids..
Before spamming pjys.org with anything commercial in this intensely well studied field you have a LOT to prove, your website shows lots of diesel engines !

1. Which method solar to H2 & what conversion efficiency & what cost per cubic meter at NTP ?
2. Do you claim your diesels pictured have been converted to hydrogen ?
3. What cost per delivered length of hydrogen piping for what delivery rate ?
4. What are your costs for an engine per Kw to run off hydrogen ?
5. Where are you located ?
6. Please show just one example of a site install ?
7. Testimonials from customers ?
8. What type of applications or just electricity generation, if so why not use solar directly ?

You have to provide a lot more than a link to a badly written website that looks like leader for $
phprof
1 / 5 (1) Apr 05, 2015
This researcher is an idiot. If you want reliable power I suggest nuclear fusion. Anything less is going back to the past. Then again, I bet this guy would love that. Puts him and his cronies in position to be part of the elite class with the serfs under their thumbs.

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