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I. THE EU ENERGY POLICY ON RENEWABLE ENERGY SOURCES
The climate change and associated consequences have been the driving force behind energy policies in every region in the world over the last decade! The broad target is towards reducing CO2 emissions in the atmosphere to levels detected in the 1990s [EU Energy Policy to 2050]. The electrical power grid is seen as pivotal to achieving this target. Thus, de-carbonizing the energy supply is imperative.
The EU in particular is keen on having the electrical power grid 100% reliant on RES (Renewable Energy Sources) by 2050, and have outlined decade-wise steps towards achieving targets of de-carbonizing the energy supply system. One bold step towards this target is that, “no new carbon-emitting power plant should be built post 2015 in the EU (European Union)”. Currently, there is a reluctance from energy investors to invest in conventional systems (fossil-fuel based systems). Thus, the share of RES is increasing and is expected to increase further as a deliberate results of environmentally friendly policies.
Notwithstanding the promising nature of RES, their integration into the conventional power grid is currently done “conservatively” in order to maintain expected system reliability and security. There is a technical limit to which the current system can withstand increasing penetration of RES owing to some challenges of RES.
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II. THE CHALLENGE OF RENEWABLE ENERGY SOURCES
It would be gravely naive just to think that as much RES as possible can be integrated into the conventional fossil-fuel based power grid. Have had several questions asked from a non-technical audience such as; “why not just connect more solar and wind since it is available?”, or more frequently, “why not just disconnect the carbon emitting power plant?” For this someone would have to play the “devil’s advocate” so we can have an unbiased picture of the challenges ahead. I have to make clear at this juncture that this is not to paint a bleak picture of RES, but to give an idea of the challenges at hand in a clear frame; solutions do exist, some are currently being implemented and more have been proposed to some of these challenges, and is the main direction of this post. There are much more challenges than what is described in this post. This post mainly focuses on technical challenges. A little background of the challenges are given in brief.
Taking Europe as a reference continent. There are a few major RES for grid level integration; hydro, wind, solar, and biomass. Others such as hydrogen fuel cells, and energy storage technologies are promising, but still not at grid scale.
The north (the Scandinavian in general) has enormous hydro resources. Norway in particular, supplies 95%-98% of its electricity consumption from hydro turbines. In fact, Norway and Switzerland are expected to act as the energy storage of the central EU grid. But is this enough to support the EU grid, and who will provide supply to them? The west has enormous sparsely distributed wind resource; what happens when there is a low wind speed for 14 consecutive days? (A grossly conservative number, it could be worse!). The east has a promising biomass resource, but this is a seasonal resource; what happens off season? The south has a lot of solar energy with a mix of wind, but in a broad sense, even in the region with the highest sunshine hours/day in the world, sunlight is at most available for less than 12 hours/day; what happens during the remaining 12 hours? We can describe all the previously mentioned challenges under long term intermittence and geographic sparsity of resources.
It is clear from the above challenges that one means of solving this problem (as have been proposed) is to install RES in as many geographic locations as possible, and as sparse as possible. This is because the profile of a resource is not the same across any single region.
Another proposed solution (for which is the main theme of this post) is to connect all resources across the previously mentioned European regions in a Pan-European network for power exchange. This is an adept and highly efficient solution aimed at negating the effects of geographic sparsity and long term intermittence, and it is just logical to share resources. For instance, hydro resources from Norway can be shared with Italy on a day with low solar irradiance; whilst solar power can be shared with Norway on a day with surplus solar resource in order to keep the balance. Therefore, no region will have to single handedly bear the cost and more aggregated investment can be made, thus high reliability and security achieved.
The above described is currently the major solution to large scale adoption of RES, but it is by no means an easy feat. Like every other innovative technological solutions, “law of conservation of misery” still holds. The previously described challenges are challenges that almost anyone would identify with “prima facie”. However, most of the challenges that would be mentioned in the following (and for which my work is based on) are those that are not obvious to a layman and are the results of a “trickle-down effect” of the last proposed solution of interconnecting resources. These are mainly technical challenges, considering the solution in the previous paragraph.
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III. THE DILEMMA OF POWER ELECTRONICS FOR SUCCESSFUL INTEGRATION OF RES
Again, going by the previously mentioned solution, long distance interconnections are envisaged (in the range of 500km and above) in solving the challenge of long term intermittence and geographic sparsity of renewable sources. Looking at the map, there are already short distance interconnections between close countries called the point-to-point connection. However, this is not a reliable method. Loss of one terminal in such a connection, implies complete loss of energy supply between the two connected terminals. Therefore, a meshed HVDC (high voltage direct current) grid is envisaged, similar to meshed AC (alternating current) grids as depicted in the expected map of interconnections by 2050.
On the other hand, short term intermittence or RES can easily threaten the security of electricity supply. To understand this clearly, the current AC system from generation level to consumer level is a frequency dependent system and balance amongst other concepts is required as explained in a previous post on low inertia grids.
Advancements in power electronics have made it possible to convert an intermittent resource (variable AC frequency resource) into direct current (DC) with zero frequency at one stage (like a neutralization) and back to AC of constant frequency at 50/60 HZ at another stage. More so, solar, several other resources, and batteries generate DC which have to be connected to AC, thus an inversion with power electronics is required. The same goes for the envisaged long distance interconnections. AC transmission beyond 500 km becomes unbearably costly, reliability and stability issues start to arise particularly in peak periods, and the system becomes unnecessarily complicated. With DC, there is no limit on transmission distance (theoretically), stability is improved, and reliability is higher. Once again, power electronic converters have found use.
Think of a high power converter as a really big laptop charger. A laptop charger is in fact a power converter, but is called a charger because its main function is to charge the battery which is a DC device. A high power converter station ranges from half the size of a football field to a full field (50-100m). The only difference from a laptop charger is that it can convert from AC to DC and vice versa; AC-AC (from one frequency to another); or DC-DC (from one voltage level to another); but a typical charger just converts from AC to DC.
Therefore at this point in time, if the EU target of 100% dependence on RES is to be taken seriously, we can justify an assumption that the future integration of RES into the grid will almost entirely (at least 65% of available RES) be via a power electronic converter. Besides RES, newer electrical systems such as electric vehicles, and storage technologies are expected to be interfaced to the grid via power electronics.
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IV. STRENGTH VS. RESILIENCE IN POWER ELECTRONIC INTERFACED RES
The strength of a power system cannot be overemphasized and has for decades been a topic of debate in power engineering parlance. This became more intense when the prospect of power electronic converters for grid scale integration of RES and energy storage came into play about a decade ago. This article has more on the consequences of strength or lack thereof.
The strength of a power system is in general a measure of a variable referred to as the aggregated inertia constant or simply the inertia constant. Each synchronous machine in the system has its own inertia constant proportional to its weight and speed of rotation; it can also be expressed in seconds (s), and it is a “fail-safe” parameter guaranteed by a manufacturer. Each machine in the power system contributes to the aggregated inertia of that system. The higher the inertia, the “stronger” the system (in principle). Inertia is a physical property of any object that has weight, can be put into motion, or brought to a stop. Thus it is that property of a body to resist change while in motion or at rest. This is synonymous with the power system. A machine with high inertia can store more energy to resist a change (typically a disturbance) while in operation and so does a system with high inertia. In theory, a machine or system without inertia cannot resist any disturbance.
Now that we are clear on what strength of a power system is, we go back to our power electronic converter. As previously mentioned, your laptop charger has no rotating part! It’s all solid-state static electronic components with single components that weigh less that 10g (except for the transformer which is arguably the heaviest component and chargers no longer require such any more). Now let’s scale up the charger to a high power level in the MW range. Size changes, but it is still a static device with no rotating part and weighs orders of magnitude less than one synchronous machine (compared to system that has 100s of generators), and cannot store as much energy as a rotating machine.
ABB’s HVDC Light Converter Hall (www.offshorewind.biz)
This is the challenge with power electronic converters. They are inertia-less systems and based on the definition of the strength of a system, 100% RES dependent system will have no strength, as such cannot resist disturbance (or slow down the effect of disturbance for control systems to act). This translates to complete loss of reliability and constant insecurity of supply. May be the concept of reliability may sound vague and even a typical consumer will confuse the amount of reliability they think they want, and what they actually want; so I will give a clear picture of what reliability means.
A typical consumer may naively accept that 99.9% reliability is sufficient or acceptable. However, in an 8760 Hours/year (non-leap year), 99.9% reliability translates to 8751 Hours/year of energy supply. This implies it is acceptable to be 9 hours without electricity supply in a year — I don’t think a paying customer would accept this. With this bit of extra information the same customer that accepted 99.9% no longer accept what it really means. The actual acceptable reliability will be in the rough edge of 99.99% which translate to 8759 Hours/year of electricity; a typical consumer would think the possibility of just 1 hour without electricity supply in a whole year is a fair deal.
What does this all have to do with dynamics of future power systems? We are getting there.
The point is, the system dynamics is already changing for the worse with current levels of penetration of RES. With power electronic interfaces including from RES, energy storage technologies, electric vehicles, the dynamics are expected to get worse. Notwithstanding all these challenges, power electronic converters will dominate future power grids. Hence, something must be done.
So what can we do? We know power electronic interfaced RES weakens the current system and would further weaken the system as more RES is integrated, leading to loss of reliability. We cannot do anything to improve their strength and the eventual strength of a system dominated with converters. But we can do something to improve the resilience of a converter dominated power system. Power electronics converters have an advantage — controllability and flexibility. As they are inertia-less, they respond to control very quickly. This can be used to an advantage. Thus, if we know the kind of disturbance to expect, or the accompanying phenomena during a disturbance, we can design control systems to quickly mitigate the disturbance, and bring the system to a new stable point, ensuring stability and reliability. We cannot do so much about the strength of a converter dominated grid (until inertia is no longer a measure of strength), but we can improve the resilience of a converter dominated power system.
I think now is the right time to ask — why this work? Simple but broad answer; to ensure a high reliability and security of supply in a grid with high penetration of RES up till 100% using innovative control strategies and systems.
