A Comprehensive Review on Power System Flexibility: Concept, Services, and Products

Massive proliferation of Variable Energy Resources (VERs) in modern power systems has posed a variety of challenges to the reliable operation of the power grid and has, at times, jeopardized the system flexibility. Flexibility is the system’s ability to respond to and cope with the imbalances between supply and demand while managing the uncertainty and variability of VERs and maintaining the power system’s security and reliability within the acceptable margins. Leveraging the system’s available resources and capabilities, the system operators must take strategic actions to mitigate the impacts of VERs on the grid flexibility at a reasonable cost. The concept of flexibility is somewhat novel, which calls for profound studies and analyses to address different aspects of flexibility, but not limited to definition and characterization of standard metrics and indices to measure the power grid flexibility, flexibility-centred operation and planning models for the power grid, etc. This paper provides a comprehensive review of the state-of-the-art research on power system flexibility, including existing definitions and quantification measures, flexible resources, and flexibility products and services in electricity markets.

the system capability in terms of frequency control services; 29 however, generation variabilities in intra-hours involve the 30 ability of supply to follow demand and operational reserves' 31 capacity appropriation. The deeper the penetration of VERs 32 in a power system, the more drastic change in the shape of the 33 net-load (load minus VER). High penetration of the VERs 34 makes it extra arduous to maintain an equilibrium between 35 supply and demand since larger VER deployment comes 36 with greater rate of variability and superior forecast errors 37 on the generation outputs. These challenges have led to an 38 introduction of the term ''flexibility'' in power systems with 39 the goal to maintain economic and secure operation while 40 coping with increasing levels of fluctuations imposed by 41 extensive integration of non-dispatchable renewables [3]. It is 42 inferred that the provision of flexibility depends on the net-43 load characteristics. Foremost features of the net-load which 44 influence the system flexibility are net-load variation rate, 45 92 With the increasing integration of VERs in energy systems, 93 the concept of flexibility has been receiving more atten-94 tion in recent years. Experts in this area have tried to pro- 95 vide all-inclusive definitions on this concept. Based on [12], 96 flexibility defines a power system's aptitude to manage the 97 variability and uncertainty in both demand and supply, while 98 preserving an acceptable reliability echelon at an equitable 99 cost over various time scales. Another explanation in [13] 100 declares that flexibility should acclimatize to numerous con-101 ceivable situations at a defined marginal price. Declaring 102 additional explanation on this concept, flexibility stands for 103 a proficiency that consistently and lucratively handles the 104 net-load forecast errors through different timescales [14]. 105 Although these studies have provided respectable definitions 106 on the concept of flexibility, the key role of the definition is 107 missing in the literature. The authors in [15] proposed that 108 the time prospects in which flexibility is provisioned needs to 109 be defined properly, and the flexibility needs to be discussed 110 in these time scales. So that, the time scales need to be 111 known to realize the suitable perception of flexibility. On this 112 basis, Milligan et al. [16] portrays time scales for flexibility 113 to be seconds (inertia response as a barrier in opposition to 114 system frequency disproportions [17]) to multiple years (sys-115 tem planning prospect  120 Another definition in [18] describes deliverable energy 121 flexibility equal to the total flexibility which is obtainable to 122 propose to daily energy markets while disregarding endan-123 gering the technical constraints in the distribution system. 124 The definition disparities can be observed in and attributed to 125 the differences in the operation of energy systems facilities. 126 Hence, the potential in providing flexibility should be discov-127 ered in each sector. Besides the existing concept of flexibility 128 introduced and discussed earlier in the literature, another 129 form of flexibility can be investigated within the heating sec-130 tor of the energy systems, namely thermal flexibility. Thermal 131 flexibility is mainly obtained from flexible heat generators, 132 interconnections, and the combination of heat generators and 133 thermal storage units [19], [20]. Accordingly, flexibility may 134 be described as a system's ability to remain functionable amid 135 rapid fluctuations and manage all system components so as 136 not to surpass their operational constraints, while employing 137 all of its infrastructure's potential in all time perspectives, 138 such as from seconds to multiple years, without accruing 139 additional costs to the system's owner(s).

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Traditionally, power systems are considered flexible if the 143 operators implement Ancillary Services (AS) to handle sud-144 den contingencies, such as unexpected generator or transmis-145 sion line failures, and real-time supply-demand inequities due 146 to erroneous projection of the demand. As a requirement, 147 power system operators consider some amount of reserved 148 capacity to afford regulation AS. This capacity is managed 149 by Distribution System Operators (DSOs) and Transmission 150 System Operators (TSOs); besides, this capacity is employed 151 to recover the power system in case of imbalances via Fre-152 quency Containment Reserves (FCRs) and to reinstate the 153 frequency back to its nominal rate [21]. In modern energy 154 systems with massive integration of VERs, however, power   is not compromised by market ineptitudes.

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• have a well-planned transmission network to ensure the 173 flexibility is not only available but also deliverable [24].   bility, and privacy, among other benefits [30]. Moreover, 205 generators' ramping up and ramping down will append an 206 additional cost to the system, which could be avoided by 207 strategic utilization of ESS [31]. When concerned with VER 208 generation, the ESSs may be employed to assist higher VER 209 penetration by extenuating their impacts on the grid oper-210 ation [32]. VERs penetration effects are characterized in 211 different time horizons ranging from seconds to years.

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At the time scales of seconds, the ESS can provide inertia 213 in case of sudden power fluctuations [33], thereby relaxing 214 the role of generators in terms of system frequency response 215 services [34]. Inertia, in a comprehensive sense, denotes the 216 kinetic energy stored in the rotating mass of the synchronous 217 generators to compensate the frequency deviation from its 218 nominal value when huge disturbances occur Demand response (DR) program or demand-side flexibility 246 is used along with ESSs to mitigate the concerns VER may 247 impose to the power grid. DR programs enable shifting of the 248 demand pattern to cope with the mismatch between demand 249 and supply [46]. DR is a promising approach enabling elec-250 tricity customers to adjust their energy consumption sub-251 jected to financial incentives or long-term agreements [47]. 252 In modern energy systems, these customers can be new 253 energy system structures, namely energy hubs which can 254 run a price-based demand response model based on energy 255 market elasticity [48]. DR usage can be spread over providing 256 AS, e.g., regulation services. A major challenge in DR is the 257 coordination of the loads in distribution grids with different 258 voltage levels. In particular, the coordination of various-size 259 loads may hurdles achieving the expected frequency response 260 rates of the system and reaching reserve capacity reduction 261 goals [49], [50]. Aggregators contract with energy customers 262 VOLUME 10, 2022 As depicted in Figure 1, the FRP ensures ramping availability 302 to encounter divined net-load from its prediction uncertainty. other market product offers resource convenience in serving 317 as power supply, in grid contingencies. This guarantees per-318 manency of grid's reliability with the provision of sufficient 319 resources to satisfy imminent energy needs. Midcontinent 320 Independent System Operator (MISO) is an example for 321 dealing with such market service, in which providers enjoin 322 the market by increasing power production and reducing 323 the power consumption. Contributors include new genera-324 tors, retrofitted and enriched generators, DR aggregators, and 325 Regional Transmission Enhancement Planners (RTEP) [61]. 326 Generally, market products are derived from some policies 327 that are potential solutions to enhance the power grid flex-328 ibility e.g., using the available resources, specifically, ramp 329 capacity of dispatchable power plants, rapid response ESSs, 330 spinning reserves, DR, power facility reinforcement, and 331 launching new flexibility products in the electricity market in 332 order to holistically provide certain levels of flexibility [62], 333 [63], [64].

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As stated earlier, the flexibility options are established to 336 maintain a cost-effective system. The provision of the flex-337 ibility in different power system sectors is recently been 338 more imperative while nations are moving toward reduc-339 ing emissions and Net Zero Energy (NZE) programs [65]. 340 In this manner, higher proportions of VER are anticipated 341 to raise the value of options to increase the flexibility of 342 the energy systems, while reflecting economic advantages 343 of energy system flexibility alternatives is essential [66]. 344 A comparative analysis by the scholars in [67] show that 345 demand response application (demand side), retrofitting the 346 thermal units (supply-side), ESSs, and establishment of new 347 grid facilities and interconnections are economically and 348 technically sorted from top to bottom, respectively. Relative 349 studies by [68], [69] show that as climate goals are modest, 350 demand side flexibility options provision has a significant 351 influence on system costs, but sector coupling with the district 352 heat sector and investment in grid facilities have a growing 353 impact among other flexibility options, when climate tar-354 gets are more ambitious. As a result, the sector coupling 355 needs accurately assessing the storage requirements, which 356 charge [70]. It can be concluded that depending on the cli-  Table 1.  systems is introduced to enhance the thermal flexibility of the 475 CHP units through participation in reserve services. STOR 476 is offered in case of generation surplus or shortage. STOR 477 providers must be available all throughout market openings 478 i.e., daily hours during which the supply margin is expected 479 to be tighter and, upon request, they must be able to deliver 480 full active power for up to 2 hours [92]. Other short-term 481 operating reserves, different from STOR category, are high-482 lighted in Figure 3, e.g., Enhanced Operation STOR [93], 483 STOR runaway [94] and, Balancing Mechanism (BM) start-484 up [95]. Demand Side Balancing Reserve (DSBR) provides 485 supplementary reserve in order to balance the unlikely sit-486 uations in which there is insufficient capacity to meet the 487 demand. DSBR concerns large energy users who could vol-488 untarily reduce electricity consumption, solely in the winter 489 period, with a payment in return [96]. Supplemental Balanc-490 ing Reserve (SBR) provides generating capacity; however, 491 DSBR provides an opportunity for major energy consumers 492 and aggregators to get paid in return for their contributions to 493 moderating the energy consumption during peak times [97]. 494 SBR is utilized when all market-based actions exhausted, 495 where generators are dispatched in economic order (i.e., uti-496 lization price and duration required). DSBR and SBR, together, present Contingency Balancing 498 Reserves (CBR) as a transitional product to regulate the 499 grid frequency during contingencies. Industrial and commer-500 cial entities which can structure the rules of AS programs 501 can serve as DSBR providers. Energy system operator in 502 National-grid ESO is no longer procuring this service, as they 503 transition to capacity markets. Management is procured for efficient and economic operation 523 of the power transmission system [100], [101], [102]. and the associated inefficiencies. According to [58], [103], 533 [104], [105], [106], three general categories on the existing 534 methods are introduced i.e., visualized methods, metrics and 535 comprehensive models. Visualization methods, e.g., illustra-536 tion of dynamic upward and downward ramping capability 537 curve [107], [108], are easy to understand but need lots of 538 information. Comprehensive methods [109], [110] are widely  [111], [112]. 545 Operational flexibility can be defined in different time 546 scales, including long term, long to medium term, medium 547 to short term and short to very short term [113]. A brief 548 schematic with analytical solutions and corresponding high-549 lights is depicted in Figure 4. Experiments have shown that 550 the planning timeframe is the most economically advanta-551 geous way to take flexibility into account [114]. Considering  impacts on net-load uncertainty and variability in hours and 569 intra-hours prospects, market operations should be analysed 570 in real time. The necessity of day-ahead decisions comes 571 from the fact that hourly scheduling protocols in real-time 572 operation are insufficient to provide system operators with 573 the required flexibility to manage their system effectively 574 [118], [119]. Designing intra-day markets is crucial in terms 575 of leveraging the full flexibility potential of the power grid. 576 Besides, setting a shorter span between gate closure and 577 actual market transactions can notably enhance the flexibility 578 in this timescale. According to [6], it can be justified that 579 this setting is preferred as the changes in VERs, especially 580 in wind, could be extremely large in real-time, which is 581 not reflected in the offline sight (day-ahead) of the system 582 operation. This vision concerns the timescale of the sys-583 tem operation which directly alters the system flexibility. 584 In minutes to seconds, utilization of AS is necessary for reli-585 able and flexible grid operation, particularly compensating 586 unexpected imbalances between demand and supply. Reg-587 ulators must introduce new products and deploy operating 588 reserves to incentivize flexibility providers to partake. Gen-589 erally, an ingenious service launched into practice by several 590 ISOs e.g., National Grid ESO, is the Fast Frequency Response 591 (FFR) delivered by BESSs and VERs [120]. Flexibility is pro-592 visioned in PJM, CAISO and EPEX in day-ahead, intraday, 593 and the intraday continuous market auctions [121].

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Most recently, the research community has recognized the 596 need for flexibility in power systems while renewable energy 597 sources are being aggressively integrated worldwide. High-598 lighting the latest updates on the concept of flexibility, this 599 paper tried to epitomize the required information about the 600 electrical and thermal flexibility and the services in which this 601 concept is impudent. This research focused on unifying the 602 available meanings and metrics of flexibility and introducing 603 the markets wherein the flexibility is traded in the form 604 of power system utilities options. It should be noted that a 605 unified framework to evaluate power systems flexibility and 606 VOLUME 10, 2022 demonstrate it with a global index is yet to be addressed.