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Viability of providing spinning reserves by RES in Spanish island power systems

2021; Institution of Engineering and Technology; Volume: 15; Issue: 13 Linguagem: Inglês

10.1049/rpg2.12216

1752-1424

Mohammad Rajabdorri, Lukas Sigrist, Enrique Lobato, Maria del Carmen Prats, F.M. Echavarren,

Frequency Control in Power Systems

IET Renewable Power GenerationVolume 15, Issue 13 p. 2878-2890 ORIGINAL RESEARCH PAPEROpen Access Viability of providing spinning reserves by RES in Spanish island power systems Mohammad Rajabdorri, Corresponding Author Mohammad Rajabdorri mrajabdorri@comillas.edu orcid.org/0000-0002-4042-7442 IIT, School of Engineering ICAI, Universidad Pontificia Comillas, Madrid, Spain Correspondence Mohammad Rajabdorri, IIT, School of Engineering ICAI, Universidad Pontificia Comillas, Madrid, Spain. Email: mrajabdorri@comillas.eduSearch for more papers by this authorLukas Sigrist, Lukas Sigrist orcid.org/0000-0003-2177-2029 IIT, School of Engineering ICAI, Universidad Pontificia Comillas, Madrid, SpainSearch for more papers by this authorEnrique Lobato, Enrique Lobato IIT, School of Engineering ICAI, Universidad Pontificia Comillas, Madrid, SpainSearch for more papers by this authorMaria del Carmen Prats, Maria del Carmen Prats IIT, School of Engineering ICAI, Universidad Pontificia Comillas, Madrid, SpainSearch for more papers by this authorFrancisco M. Echavarren, Francisco M. Echavarren IIT, School of Engineering ICAI, Universidad Pontificia Comillas, Madrid, SpainSearch for more papers by this author Mohammad Rajabdorri, Corresponding Author Mohammad Rajabdorri mrajabdorri@comillas.edu orcid.org/0000-0002-4042-7442 IIT, School of Engineering ICAI, Universidad Pontificia Comillas, Madrid, Spain Correspondence Mohammad Rajabdorri, IIT, School of Engineering ICAI, Universidad Pontificia Comillas, Madrid, Spain. Email: mrajabdorri@comillas.eduSearch for more papers by this authorLukas Sigrist, Lukas Sigrist orcid.org/0000-0003-2177-2029 IIT, School of Engineering ICAI, Universidad Pontificia Comillas, Madrid, SpainSearch for more papers by this authorEnrique Lobato, Enrique Lobato IIT, School of Engineering ICAI, Universidad Pontificia Comillas, Madrid, SpainSearch for more papers by this authorMaria del Carmen Prats, Maria del Carmen Prats IIT, School of Engineering ICAI, Universidad Pontificia Comillas, Madrid, SpainSearch for more papers by this authorFrancisco M. Echavarren, Francisco M. Echavarren IIT, School of Engineering ICAI, Universidad Pontificia Comillas, Madrid, SpainSearch for more papers by this author First published: 26 May 2021 https://doi.org/10.1049/rpg2.12216Citations: 1AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract This paper assesses the viability of providing down and up spinning reserves by renewable energy resources (RES) in island power systems. The process consists of evaluating the impact of providing spinning reserve on the system operation costs of different islands by simulating the unit commitment problem. The assessment is carried out for La Palma (small size) and Tenerife (medium size) island power systems, and by considering different wind source availability scenarios for sample weeks of different seasons in current and future years. This paper differentiates between up and down reserves and studies their impacts separately. Results show that enabling RES to provide just down spinning reserve has economic benefits for all scenarios, by reducing over 40% the amount of thermal generation and over 30% the systems costs for high wind scenarios. It also confirms that employing variable deloading of wind energy as a source of up reserve is advisable, mainly in scenarios with high share of wind sources. In some scenarios, using RES as reserve provider, reduces the amount of thermal generation more than 50%, compared to when RES does not participate as a source of reserve, and can even lead to a full RES coverage of demand. 1 INTRODUCTION Islands are facing considerable challenges in meeting their energy needs in a sustainable, affordable and reliable way. This is mainly due to the isolated nature and the small size of island power systems. The geographic isolation also causes relatively high operation costs in comparison to large interconnected systems. Operation costs are not only higher because of expensive fuel transportation and lower efficiencies of the power generation technologies (e.g. Diesel), but also because of technical spinning reserve requirements to guarantee frequency stability. Actually, island power systems are more prone to suffer from frequency instability than larger interconnected systems, since they poses a smaller inertia and each generating unit represents a significant fraction of the total generation in-feed [1]. According to local resource availability, renewable energy sources (RES) offer an interesting solution to decrease the dependency on fossil fuels and increase island sustainability [2]. In [3], the possibility of achieving 100% renewable generation in Canary Islands before 2050 is investigated. [4, 5] determine the potential of off-shore wind generation and solar PV roof-top installations in the Canary Islands. In current practice by operators, all available RES generation is directly injected to the power system, substituting thermal generation [6]. However, the increasing penetration of RES can negatively affect frequency stability of island power systems even further [7, 8], by reducing control capacity and system inertia. Spinning reserves denote the sufficient power and energy reserves to contribute to frequency stability. Spinning reserves in power systems should be able to cover both emergency and non-emergency conditions. Nonemergency incidents include expected RES fluctuations (wind and solar forecast error) or the demand variations (demand forecast error). And emergency incidents include for instance the loss of generation units in case of generator trips or transmission line outages [9]. By increasing the injection of uncertain renewable power into the system, more reserve is required to balance the forecasted generation and real time demands, hence an adequate sizing of the reserve is essential [10]. Refs. [11, 12] study the provision of reserve margins to hedge against real-time uncertainty and variability of wind power generation. The impact of forecasting horizon and amount of RES generation on reserve requirements has been analysed in [13]. However, RES does not provide spinning reserves so far in Spanish island systems. RES generation can be curtailed to ensure system stability, when over-generation is about to happen. When RES provides no spinning reserves, there should be some thermal generation above minimum power to serve as down reserve and same or different thermal units should keep some headroom below maximum power to serve as up reserve. Thus, RES providing down and up reserve can change the commitment status of units to reduce the operation costs. Since enabling RES to provide up reserve is completely different with enabling them to provide down reserve and they change the commitment status differently, they should be studied separately. As far as the authors' knowledge, this hasn't been done in previous studies. Synchronous generators have always been the main providers of inertia and frequency regulation in the power system. Non-synchronous RES are unable to increase the inertia of the system unless appropriate controls are in place, because the converters decouple them from the grid [14]. Researchers have been trying to find ways of enabling wind turbines to contribute in primary frequency regulation and deliver inertia to the system. In [15], various reserve allocation methods are compared and a practice to assess immediate wind primary reserve is presented. Ref. [16] has tested various control strategies of active power to investigate their effectiveness in times of high wind injection. It concludes that inertial and power frequency response controllers can be implemented on wind turbine generators and enhance the overall frequency response of the system. In [17] an aggregated frequency response model for wind generators is presented, considering the different operational modes of wind power turbines. Then an analytical approach is employed to aggregate low-order frequency response model of all wind power plants into one model. In [18] a stochastic unit commitment formulation is proposed, to evaluate the advantages of synthetic inertia and primary frequency response provision from wind turbines in Great Britain power system and concludes that it potentially can mitigate operation costs of the system. Ref. [19] has mentioned some inertia and frequency regulation approaches for RES: Deloading techniques, inertia emulation, fast power reserve, and droop techniques. Among them, deloading brings more economic and technical benefits and provides a better frequency response [20]. Although deloading practice enables wind turbines to take part in frequency regulations, it contradicts with the principle of acquiring the highest possible amount of power from wind source [21]. RES such as wind power or solar generation are technically able to provide reserves by deloading a percentage of their maximum power point tracking (MPPT) operation [22]. This can be achieved by appropriately adjusting rotor speed in wind turbines or the DC-link voltage in photo voltaic systems. Typically, deloading rate is less than 20% of the actual available RES power, depending on the circumstances [23]. An extensive review on deloading of wind turbines in power systems is presented in [24], and different control modes are compared. A stable operation of wind turbine generators is introduced in [25], which guarantees the optimum contribution of each wind turbine to improve the primary frequency response of the system. A dynamic strategy of active power control is presented in [26], to maximize the role of variable speed wind turbine in primary frequency regulation. The authors employ a fuzzy control method to sense the frequency deviations and adjust the amount of deloading subsequently. In [27], the authors argue that existing linear deloading techniques lack accuracy, and the nonlinear relation between rotor speed and output power during deloading practice should not be overlooked. Then they've proposed a nonlinear formulation to enhance stability and frequency regulation participation of wind turbines in micro grid. The reviewed literature is summarised in Figure 1. They've been classified depending on their issues. The ones that are particularly related to islands are highlighted with dashed lines. Those that are applied to real systems are specified with double arrows. FIGURE 1Open in figure viewerPowerPoint A summary of the reviewed literature. Those that are applied to real systems are specified with double arrows According to the Canary Islands Energy Yearbook of 2018 [28], there's been 9282.8 GWh of annual energy production, consisted of around 10% renewable generation, 90% thermal generation and less than 0.01% refinery and cogeneration in 2018. Only in 2018, the amount of 1819.8 kilotonnes of fuel (including gasoil, diesel oil and fuel oil) has been imported to Canary Islands, for the purpose of electricity generation. They're planning to add 200% to the renewable resources by 2025, and add 400% renewable capacity by 2030. Under such scenarios, the question arises whether reserve should be still provided by synchronous generators only or whether non-synchronous RES should participate as well. For this purpose, the islands of Tenerife (medium size) and La Palma (small scale) are chosen for simulations because they are representative for the Spanish isolated systems. Further, the results shown here can be extrapolated to other islands to a good extent, since these two islands seem to fit in two of the five prototypes islands identified through clustering techniques in [29]. The main objective of this paper is to evaluate the contribution of providing up and down spinning reserves by RES generation. The assessment consists of determining the impact of providing spinning reserve on the system operation costs by simulating its economic operation. As most island systems are operated under a classical centralized scheme, hourly unit commitment (UC) on a weekly basis is proposed for this purpose. The focus is on analysing whether reserve provision by RES generation is beneficial, whereas the actual implementation of the corresponding operation planning is out of scope. The actual implementation is affected by the variability of RES and might require operation planning methodologies under uncertainty, but to highlight the economic aspects of providing reserve by RES, a deterministic approach considering different scenarios (seasons and years) would be sufficient. To contribute to the previous publications, the methodology of this paper is applied to two real islands, La Palma and Tenerife, with factual input data. Four different approaches of reserve provision are considered (RES providing no reserve, RES providing only down reserve, RES providing up and down reserve with a fixed constant deloading factor and RES providing up and down reserve with a dynamic optimum deloading factor) are considered as different cases and are applied on various seasonal scenarios (summer, autumn, winter and spring), both for current and future timeframes (years 2020, 2025 and 2030). Up reserve and down reserve are formulated and analysed separately, then the impacts of each on the operation cost is included. Deloading is defined as a variable in UC problem, and the amount of deloading is optimized for each hour in the last approach of RES reserve provision. A total number of 240 UC weekly simulations are performed for each island. Note that this paper tackles the economic benefits of RES up and down reserve provision. The technical benefits of reserve provision on the dynamic frequency response performance will be analysed in future research. However, the non-synchronous RES can deploy reserve faster than conventional synchronous generators. The rest of the paper is organized as follows. Section 2 summarises the regulations of Spanish isolated systems. Then in Section 3, the methodology used in this paper is explained. In Section 4, the unit commitment formulation of the optimization problem is introduced and the corresponding constraints are presented. In Section 5, the obtained results for both islands are fully analysed. Conclusions are drawn is Section 6 and after that acknowledgments, nomenclature, and references are presented. 2 REVIEW OF THE REGULATION OF SPANISH ISOLATED POWER SYSTEMS This section provides a short review of the regulation of Spanish isolated power systems [30]. The Spanish isolated power systems are the power systems of the Canary Islands, Balearic Islands and the Spanish towns in North Africa. These systems are of very different sizes. The largest system is Mallorca-Menorca system with a peak demand around 1100 MW and the smallest system is El Hierro system with a peak demand of 7 MW. 2.1 Reserve requirement The technical regulatory framework of the Spanish isolated power systems is defined in a set of operational procedures [30]. Among others, the operation procedure number 1 describes the spinning reserve requirements in the isolated Spanish power systems. It points out that the up-spinning reserve, including primary and secondary frequency control reserves, should be greater than the largest online unit, greater than the expected RES power generation variations, and greater than the largest interconnection infeed. In addition, down spinning reserve must be at least 50% of the upward primary reserve. The operational procedure also recognizes that during the outage of a large unit, primary frequency control makes use of both primary and secondary reserves. 2.2 Economic regulation Isolated power systems can be operated either under a classical centralized scheme or under a market driven scheme. Spanish isolated power systems are operated under a centralized scheme. In a classical centralized scheme generating units are programmed according to economic dispatch rules that consider security of supply. Generation program is sequentially determined over different time horizons: weekly, daily, intraday and real-time. The weekly generation program is initially determined by a UC and security of supply criteria. The UC contemplates standardized variable operation costs. In a second step, technical restrictions of the network are imposed and generation units are re-scheduled if needed. Determination of the daily generation program is similar to the determination of the weekly program. Generators in Spanish isolated power systems are divided into two categories: category A includes hydro (excluding run of the river) and thermal generators and cogeneration power plants with net power greater than 15 MW, whereas category B refers to renewable energies and cogeneration power plants with a net power equal or lower than 15 MW. Renewable sources and high efficiency cogenerators (of both category A or B) have priority of dispatch under equal economic conditions, considering that the security of supply requirements is maintained [30]. Generators of category A that have been included in the additional remuneration scheme (regimen retributivo adicional), are remunerated according to fixed costs and variable generation costs in function of the generation technology. The additional remuneration scheme repays investments and exploitation expenditures. Generators of category A that are not included in this scheme perceive a payment according to the hourly energy selling price and the energy produced. Generators of category B are remunerated according to the hourly energy selling price and the energy produced plus a specific remuneration as well as a payment for their contribution to ancillary services (if any). Note that the hourly energy selling price of the Spanish isolated systems depends on daily or intraday market price of the mainland system weighted by the relation between actual hourly demand and average daily demand of the isolated system of interest. 3 METHODOLOGY TO ASSESS THE VIABILITY OF PROVIDING SPINNING RESERVES BY RES This section presents the methodology to assess the viability of providing spinning reserves by RES in island power systems. First, the main benefits behind the provision of spinning reserves by RES are illustrated. Second, an overview of the proposed methodology is given. The assessment is based on the simulation of the economic operation by means of an hourly UC on a weekly basis. 3.1 Illustration of the benefits of providing reserves by RES To provide spinning reserves, conventional units are connected and operated below the maximum power generation. The amount of required spinning reserves can be substantial in comparison with the total generation, increasing operation cost significantly. Operation costs could be reduced by providing spinning reserves by RES. Figure 2 illustrates the main idea and benefits in terms of cost reduction of providing spinning reserves by RES. Suppose a hypothetical power system with two conventional units G1 and G2, and two wind farms, W1 and W2, feeding a certain demand at a given instant. G1, G2, W1, and W2 have the same size in terms of maximum generation at that given instant. In Figure 2(a), the demand is covered by the generation of units G1 and G2, and spinning reserves is also provided by G1 and G2. Note that both units operate at the same power level to cover their possible individual outages. In Figure 2(b), the demand is covered mostly by the wind farm W1 but also by G1, whereas reserve is mostly provided by unit G1. Note that unit G1 operates at the minimum power generation level. In Figure 2(c), the demand is covered by the two wind farms W1 and W2, and spinning reserves are also provided by W1 and W2. Since operation costs of wind farms are usually much lower than those of the conventional generation, it is reasonable to assume that the operation cost decreases from Figure 2(a–c). FIGURE 2Open in figure viewerPowerPoint Illustration of the benefits of providing spinning reserves by RES: a) Covering demand and providing reserves by G1 and G2, b) Covering demand by W1 and providing reserves by G1, c) Covering demand and providing reserves by W1 and W2 Although the example is only illustrative and highly hypothetical, it shows the benefits of providing reserves by RES. It also insinuates that this provision makes sense under high RES penetration scenarios, where the exceeding available RES energy is not simply spilled, but reserved. The difference with respect to spilling is that an appropriate primary frequency controller is required to release the reserved energy. 3.2 Overview of the methodology The methodology is based on simulations of the economic operation of islands under different demands, RES penetration scenarios and cases with different approaches of providing reserve. The economic operation is simulated with an hourly UC on a weekly basis. The UC determines the hourly generation set point as well as the hourly start-up and shut-down decisions. For a given weekly demand profile, the corresponding current RES profiles are scaled up according to the considered future installed capacity. Scaling-up current profiles is a proxy for future profiles under higher penetration scenarios since the current installed RES and RES spillage are low. For each weekly demand and RES generation profile, the simulation of economic operation is performed, considering whether RES is controllable (the subset of controllable RES is denoted as cres in the paper) and able to provide up or down reserves or not. Different cases are considered which are introduced in results section. Figure 3 shows a flow chart of the methodology. Input of the weekly unit commitment includes the weekly hourly demand, wind and solar generation forecast, list of thermal generators and their data sheet for each island and each sample week under study. Considered scenarios are further discussed in Section 5. FIGURE 3Open in figure viewerPowerPoint Flowchart of the methodology 4 UC MODEL The UC is formulated as minimization problem where generation set points and start-up and shut-down decisions are such that the total weekly operation cost is minimized by considering technical constraints. The objective function as well as associated constraints are summarized next. A description of the full UC but without constraints related to reserve provision by RES can be found in [31]. 4.1 Objective function As stated in [31], the objective is to minimize the total operation costs, by finding the optimum start-up decisions of thermal units and their hourly generation. The objective function is: min ∑ t ∈ τ ∑ i ∈ I C i f i x · x i t + C i l i n · p i t + C i q u a · p i 2 t + C i s t a r t − u p · y i t + C i s h u t − d o w n · z i t (1)where quadratic generation cost curves have been approximated by piecewise linear functions. 4.2 Binary logic Binary logic of status of thermal units is defined in Equations (2) and (3). x i t − x i t − 1 = y i t − z i t , t ∈ τ (2) y i t − z i t ≤ 1 , t ∈ τ (3) Minimum up-time and down-time constraints are from [32]. It's further confirmed in other researches like [33], that this approach improves the solving time of UC problem. 4.3 Constraints 4.3.1 Demand balance Concerning demand balance, Equation (1) formulates that the total power generation (thermal units and wind) must be equal to total load demand. ∑ i ∈ I p i t + ∑ r e s ∈ R E S P r e s t − ∑ d w ∈ D W p d w d e l o a d e d ( t ) − ∑ r e s ∈ R E S p r e s s p i l l e d t = D t , t ∈ τ (4) Note that p r e s s p i l l e d ( t ) is the amount of spillage that is scheduled for renewable energy source. p d w d e l o a d e d ( t ) is introduced in the following. 4.3.2 Thermal technical operation Concerning thermal technical operation, Equation (5) makes sure that thermal units are generating between their maximum and minimum capability. Equation (6) imposes the ramping limitation. Any increment/decrement of power between two consecutive hours should not exceed generator's ramp up/down limits. P ̲ i · x i t ≤ p i t ≤ P ¯ i · x i t , ∀ i ∈ I , ∀ t ∈ τ (5) − R i ̲ − P i ¯ . z i t ≤ p i t − p i t − 1 ≤ R i ¯ + P i ¯ . y t , ∀ i ∈ I , ∀ t ∈ τ (6) Binary variables of start-up/shut-down are used in Equation (6), so the units are able to start up/shut down even if R ¯ i / R ̲ i is smaller than P ̲ i . 4.3.3 Wind power deloading In maximum power point tracking (MPPT) approach, all available energy is instantly used for generation. But in deloading control mode, a percentage of available energy is stored as reserve to support the system when a contingency happens. The maximum power is reduced by deloading factor in the optimization problem. However, the wind turbine should be controllable (receive set point variations). As an example, a general control strategy is showed in Figure 4 [23]. Reserves are activated through appropriate proportional and derivative frequency controls. Note that this paper does not focus on the details of control strategies, but it tries to study the economic impacts from the operator's perspective. p d w d e l o a d e d ( t ) = P d w ( t ) · D F t ∀ t ∈ τ , d w ∈ D W (7) FIGURE 4Open in figure viewerPowerPoint General control strategy of the deloaded wind turbine In other words, net RES generation can be reduced by spilling energy with respect to the available wind generation as long as it is controllable. Typically, RES generation under the current scenarios is only spilled in case of possible issues with respect to system stability (like over-generation). 4.3.4 System reserve requirement As specified by Spain regulations for isolated systems, up spinning reserve in each hour should be bigger than the maximum of the largest operating unit and the expected RES uncertainty. Also following Spain regulations, total down spinning reserve must be greater than k D R (here 50%) of the up-spinning reserve. Equations (8) and (9) compute the required up and down reserves. k R V is set to 30% in this paper. U R R t = max max p i t , i ∈ I , ∑ r e s ∈ R E S P r e s t − p r e s s p i l l e d t − ∑ d w ∈ D W p d w d e l o a d e d t . k R V (8) D R R t = U R R t . k D R , ∀ t ∈ τ (9) 4.3.5 System reserve provision Computation of upward and downward primary reserves provided by thermal units are formulated in Equations (10)–(13). r i u p t ≤ ∑ i ∈ I P i ¯ . x i t − p i t , ∀ t ∈ τ (10) r i u p t ≤ R i ¯ 4 , ∀ t ∈ τ (11) r i d o w n t ≤ ∑ i ∈ I p i t − P i ̲ . x i t , ∀ t ∈ τ (12) r i d o w n t ≤ R i ̲ 4 , ∀ t ∈ τ (13) The thermal unit should be able to accomplish active power increase or decrease in 15 min [34]. Equation (10) limits the amount of scheduled reserve to the extent that ramp-up rate of the unit allows (15 min is a quarter of an hour, so the ramp-up rate is divided to 4). Same explanation for ramp-down rate and Equation (12). Up reserves can be provided by renewable sources if final generation set point is below the available RES power and the proper control mechanism is implemented on them. Wind turbines can participate as up reserve providers, if they benefit from deloading control mechanism. There are different control strategies in the literature (see [23, 24, 26]), mainly possible by conventional PI controllers and small ROM memories to form the required look-up tables. The cost of adding deloading control mechanism, its tuning and its maintenance is ignored in the cost function. Still the objective function is able to reflect the opportunity cost of providing reserve by deloading wind generation instead of using the associated energy to cover demand. Renewable energy sources can provide down reserve if they are able to sense the frequency of the system and curtail their generation in case of high frequency. Considering the deloading wind turbines and those controllable renewable sources that can participate as down reserve providers, up and down reserve criteria are defined as following. ∑ i ∈ I r i u p t + ∑ c w ∈ C W p c w d e l o a d e d ≥ U R R t , ∀ t ∈ τ (14) ∑ i ∈ I r i d o w n t + ∑ c r e s ∈ C R E S P c r e s t − P c r e s s p i l l e d t − ∑ d w ∈ D W p d w d e l o a d e d ( t ) ≥ D R R t ∀ t ∈ τ (15) Equations (14) makes sure that the available up spinning reserve which is the summation of reserve provided by thermal units and deloading of wind turbines, meets the requirements. Equation (15) states that the summation of down reserve provided by thermal units and down reserve provided by controllable renewable energy sources, should be higher