How to Harvest the Full Flexibility Potentials of Renewables?
Inertia, Frequency, and Reactive Power Control via Renewable Sources
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In many articles I have written before, it was stressed that renewable power sources could already meet system flexibility requirements for certain time scales.
In this article I will provide deeper insight to how renewables do just that.
The main reason of writing this article is that during my stay in the climate camp of Leipzig, some people asked me which flexibility requirements renewables could already provide, and which still needed conventional power plants at the moment.
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Below we shall see that, variable renewables can already provide most of the flexibility in the shorter time scales, and only the medium-term flexibility (the residual load flexibility) still requires entirely of conventional power plants.
Review: Flexibilities a Power System Need
First, let us again look into the power system transition report of IEA, and see what types of flexibility are needed in a power system.
Status of Power System Transformation 2018
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From the chart above we can see that power system flexibility can be categorized into different time scales: the ultra-short-term flexibility, the very short-term flexibility, the short-term flexibility, the medium-term flexibility, the long-term flexibility, and the very long-term flexibility.
We know that long-term and very long-term flexibility are mostly related to system planning and scheduling. The amount of conventional power plants needed is based on the capacity value of renewable energy, which would reduce the required capacity and electricity output from conventional power plants.
As for short-term and other faster flexibility requirements, renewable sources may perform better flexibility than conventional power plants by integration with modern power electronics.
Synchronous Condensers and Synchronverters: Flexibility for Ultra-Short Term
In a power system, system inertia refers to the rate of frequency change of the system when subject to contingency event.
Traditionally, rotational kinetic energy of turbines provided the mechanical inertia that prevented the system frequency from rapid change.
During the transition we might keep these turbines and transform them into synchronous condensers. Synchronous condensers do not provide real power, but can perform many ancillary services such as reactive power control. In addition, they will still bear kinetic energy that can continue contribute to the system inertia.
On the other hand, sychronverters are becoming more and more mature. Unlike conventional inverters, these inverters can mimic mechanical inertia and provide synthetic inertia to the power system. Wind power fleets can provide additional synthetic inertia by extracting the rotational kinetic energy from the blades.
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System inertia is important, but what it can only do is to delay the impacts on the system after a contingency event occurs. After that, we still need some further stability control that can bring the system back to an acceptable steady state.
If we want renewables to contribute to the stability of frequency and voltage, we need them to perform very short-term and short-term flexibility. This includes primary and secondary frequency controls that we mentioned previously in an article, and also reactive power control (which is voltage control).
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A 300 MW solar farm in California conducted control tests in August 2016. It proved that with appropriate integration and good control, renewable sources could provide sufficient frequency and voltage control, in some cases even outperforming conventional power plants.
Operators of virtual power plants go a step further and try to control distributed renewable farms scattered around the grid, sometimes even single solar systems of a household. This complicates the integration of sensor and control signals.
Of course, if you have a huge battery such as the one in South Australia, it will be way easier to perform these ancillary services.
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Yes Renewables Provide Better Flexibility… But We Should not Overlook the Roles of Conventional Sources
Although renewable sources can gradually take the role of performing short-term and faster flexibility, during the transition period, the mass amount of conventional power sources still on-line will continue to affect system stability.
The conventional power plants are not to be overlooked. If the frequency control of these conventional sources is not precise enough, these power plants might response in a manner damaging the power system stability after a contingency event. In eastern Australia, the illy-designed ancillary service market has led to this kind of problem. The video below depicts the issue in detail.
Do note that the speaker did not blame the renewables for threatening power system stability; in contrast, she stressed how renewables were actually taking the role “baseload” and provided reliable power output during contingency events, debunking the myth of the intermittency of renewables threatening grid stability.
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The speaker also acknowledged that renewable sources provide better power control than conventional ones. Still, we need to learn more about conventional synchronous generators, and consider the negative impacts of loosening frequency control during the transition period.
“Must Run Capacity” to be Reduced
As I previously stated in a presentation, the only flexibility that variable renewables might not be able to perform at the moment is the medium-term flexibility (residual load flexibility).
Flexibility, Residual Load , & Re-dispatchment
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Nevertheless, as renewables replace conventional sources in providing ancillary services such as frequency and voltage control, the residual load flexibility of the power system will also improve.
This is because the more ancillary services provided by renewables, the less conventional power is needed to constantly remain on line for providing the same services (the “must run capacity”). This can lower the power output of conventional sources during renewable power surges, increasing the penetration potential for renewables.
Of course, when variable renewables provide ancillary services, they probably need to curtail some of their power output in the first place. But by doing so, the renewable operators will not only receive economic incentives, but also provide a more friendly environment for integration of more renewables. This is a win-win situation for both the policy maker and the renewable industry.
Therefore, the market design and dispatchment rules that allow renewables to perform flexibility to their full extent will be the key to energy transition.