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Handling Renewable Energy Variability and Uncertainty in Power System Operation

Ricardo Bessa, Carlos Moreira, Bernardo Silva and Manuel Matos

INESC TEC, INESC Technology and Science (Formerly INESC Porto) and FEUP, Faculty of Engineering, University of Porto, Porto, Portugal

Concerns about global warming (greenhouse-gas emissions), scarcity of fossil fuel reserves, and primary energy independence of regions or countries have led to a dramatic increase of renewable energy sources (RES) penetration in electric power systems, mainly wind and solar power. This has created new challenges associated with the variability and uncertainty of these sources. Handling these two characteristics is a key issue that includes technological, regulatory, and computational aspects. Advanced tools for handling RES maximize the resultant benefits and keep the reliability indices at the required level. Recent advances in forecasting and management algorithms provide a means to manage RES. Forecasts of renewable generation for the next hours/days play a crucial role in the management tools and protocols of the system operator. These forecasts are used as input for setting reserve requirements and performing the unit commitment (UC) and economic dispatch (ED) processes. Probabilistic forecasts are being included in management tools, enabling a move from deterministic to stochastic methods, which lead to robust solutions. On the technological side, advances to increase mid-merit and base-load generation flexibility should be a priority. The use of storage devices to mitigate uncertainty and variability is particularly valuable for isolated power systems, whereas in interconnected systems, economic criteria might be a barrier to invest in new storage facilities. The possibility of sending active and reactive control set points to RES power plants offers more flexibility. Furthermore, the emergence of the smart grid concept and the increasing share of controllable loads contribute with flexibility to increase RES penetration levels.

INTRODUCTION

The integration of renewable energy sources () in a generation portfolio conveys several benefits, such as a reduction in greenhouse gases emissions and in the country's dependency on imported energy, and it decreases spot prices. However, generation from RES (i.e. wind, solar, hydro, wave, geothermal, and biomass) can be variable and uncertain, in contrast to conventional generation (e.g. coal thermal plants, combined and open cycle gas turbines). Nevertheless, many power systems have had hydropower for a long time in their portfolio, and system operators (s) already have appropriate procedures for its utilization regarding the need to manage its variability and uncertainty. Note that hydropower is more flexible than other RES (such as wind and solar), in particular power plants with a reservoir. The installation of pumped storage units also facilitates water management. Conversely, geothermal generation is invariable, which might create problems because it is incapable of following load variations. The variability of hydropower, biomass, and geothermal is more apparent on yearly and seasonal timescales (run-of-river hydropower can also present daily variability), whereas the variability of wind and solar covers all timescales (including daily, hourly, and minutes variability).

At present, the penetration of wind and solar generation in many power systems has attained a high level, and this has created new challenges when operating the power system. In order to meet these challenges, the state-of-the-art encompasses new technological and computational advances for dealing with the variability and uncertainty of RES, particularly regarding wind and solar generation, since hydro variability has for a long-time been tackled in power systems.

New forecasting and decision-aid algorithms, including stochastic information, can improve the ability of a power system to cope with variable and uncertain generation coming from RES, without excessive extra operational cost while maintaining reliability standards. On the technological side, new technological advances to enhance the flexibility of conventional power plants (namely, base-load and mid-merit units) are essential. Primary frequency control provided by new RES power plants or the use of storage devices are also relevant research areas.

This article describes developments in several interdisciplinary topics related with managing high penetrations of solar and wind, and points toward research trends for the next years. First, the challenges introduced by RES (in the remainder of the chapter only wind and solar are considered) in power system operation are discussed. Then, an overview of the advances in renewable energy forecasting is presented. Renewable energy forecasts are an important input to methods for setting reserve requirements, defining the commitment schedule and performing congestion detection, which are reviewed. Consideration is also given to the electricity market role and the value of storage devices for interconnected and isolated systems. On the technological side, the importance of flexibility (from conventional generators and storage units) in power system operation is described, and some challenges and technological solutions unrelated to resource variability are reviewed, and the capability of active and reactive power control is analyzed.

THE CHALLENGES OF RES IN POWER SYSTEM OPERATION

Main Challenges

The intrinsic variability and uncertainty of RES create several challenges in power system operation and planning[1]. At every instant, generation must follow load variations in order to maintain the generation-load balance. The variable nature of RES (e.g. rapid generation ramps) represents a challenge, in particular, for systems without hydropower, as it introduces variations in the generation side that can only be smoothed within the physical constraints of the conventional power plants (e.g. ramping up and down, minimum generation limits). In general, the available ramping rates of flexible generation units and fast-starting units (e.g. hydropower) are used for accommodating this variability. Technological solutions such as control schemes for wind power active and reactive power set points smoothen the impact of variability. For example, a dispatch center for RES with the ability to control the active and reactive power output was created in Spain[2].

RES uncertainty also creates imbalances between generation and load as it is not possible to know (with certainty) the RES generation levels for the next hours/days. These imbalances originating from forecast errors are handled with additional generation capacity (which is an ancillary service). Computational algorithms such as forecasting algorithms and large-scale stochastic optimization (instead of deterministic tools/rules) have been developed for including information about uncertainty in the decision-making processes. The importance of new and advanced forecasting algorithms for RES, not only for the SO but also for wind power producers (in particular when trading wind power in the market), is shown by the proliferation of companies that sell this service[3]. Storage units can also play an important role in handling RES variability and uncertainty on different timescales.

If these new solutions are not adopted, variability and uncertainty of RES could lead to situations with high operational cost. For instance, curtailment of renewable generation during low load periods and the startup of expensive fast-starting units lead to a cost increase. Moreover, even with a perfect forecast for the next hours/days, it is necessary to schedule flexible generation units for accommodating the generation ramps.

Ela and O'Malley[4] presented a simulation framework for assessing the impact of wind power variability and uncertainty on several timescales. The results showed that the imbalance impacts increase with longer dispatch resolutions (ranging from five minutes to one hour) and with installed wind power. Assessment of the uncertainty impacts lead to the following conclusions: the uncertainty impact increases with the forecast error, but it is not significant until the forecast error reaches a threshold; large forecast errors have a significant impact on the generation costs and branch congestion of day-ahead scheduling, but not in the real-time dispatch.

The next subsection discusses challenges and solutions for aspects unrelated to variability and uncertainty of the resource (e.g. wind and solar). Nevertheless, situations with wind turbine tripping following voltage dips are a source of uncertainty and variability to the system, but they are essentially technological and not related with the natural resource.

Other Challenges and Technological Solutions

Specific technological characteristics of RES conversion systems, which do not depend on resource variability, bring also new operational challenges as integration levels increase. There are many important ancillary services traditionally provided by conventional thermal or hydro-based generation units, such as voltage and frequency control. Additionally, conventional generation units intrinsically provide inertia to the system, which is a fundamental characteristic in order to ensure its stability. The large-scale integration of RES naturally displaces conventional generation units, thus strongly affecting ancillary services...