Power Production Losses Study by Frequency Regulation in Weak-Grid-Connected Utility-Scale Photovoltaic Plants

Article Power Production Losses Study by Frequency Regulation in Weak-Grid-Connected Utility-Scale Photovoltaic Plants Jesús Muñoz-Cruzado-Alba 1,*, Christian A. Rojas 2,†, Samir Kouro 2,† and Eduardo Galván Díez 3,† 1Research & Development Department, GPTech, 26 Av. de Camas, Bollullos de la Mitacion 41110, Spain 2Department of Electronic Engineering, Universidad Técnica Federico Santa María, 1680 Av. de España, Valparaiso 2390123, Chile; [email protected] (C.A.R.); samir[email protected] (S.K.) 3Department of Electronic Engineering, University of Seville, S/N Av. de los Descubrimientos, Seville 41092, Spain; [email protected] *Correspondence: [email protected]; Tel.: +34-954-181-521 † These authors contributed equally to this work. Academic Editor: João P. S. Catalão Received: 17 February 2016; Accepted: 6 April 2016; Published: 25 April 2016 Abstract: Nowadays, an increasing penetration of utility-scale photovoltaic plants (USPVPs) leads to a change in dynamic and operational characteristics of the power distribution system. USPVPs must help to maintain the system stability and reliability while implementing minimum technical requirements (MTRs) imposed by the utility grid. One of the most significant requirements is about frequency regulation (FR). Overall production of USPVPs is reduced significantly by applying FR curves, especially in weak grids with high rate of frequency faults. The introduction of a battery energy storage system (BESS) reduces losses and improves the grid system reliability. Experimental frequency and irradiance data of several weak grids have been used to analyse USPVPs losses related to FR requirements and benefits from the introduction of a BESS. Moreover, its economic viability is showen without the need for any economic incentives. Keywords: distributed generators; battery energy storage system (BESS); frequency regulation (FR); utility-scale photovoltaic plants (USPVPs); weak grids 1. Introduction An increasing penetration of utility-scale photovoltaic plants (USPVPs) in the overall electric system requires the maintenance of their system stability and reliability. The required actions are divided into three categories depending on the time scale of grid operation [1]. First, the faster technique (sub-seconds to minutes time scale) is related to the photovoltaic (PV) plant response to grid disturbances on the point of common coupling (PCC). The second is related to load balancing which is of the order of sub-hours to days, and is controlled by the utility-grid. The third action is about future electric systems designing (order of years). USPVPs on each of these categories have a different role and should be studied accordingly. Countries around the world are demanding minimum technical requirements (MTRs) for USPVPs besides active contribution to grid stability and power quality [2–7], in order to comply with the first category of system stability actions. Requirements are not only under normal conditions but also under faulty conditions, such as low voltage, phase jump, under/over frequency, etc. Unlike the inherent electromechanical dynamics of synchronous generators, PV generators’ response based on power electronics is quite different. It needs a special attention on the grid frequency stability control actions [8,9]. Frequency regulation (FR) techniques have been studied Energies 2016,9, 317; doi:10.3390/en9050317 www.mdpi.com/journal/energies Energies 2016,9, 317 2 of 21 widely in the literature [1,9–20]. All FR methods require a reduction over the active power generation on USPVPs. Consequently, if MTRs about FR are fulfilled, the USPVPs will have to consider additional losses, and a reduction of the overall plant profit. Figure 1shows a recent literature classification of FR methods used on PV applications. Depending on the grid system size (microgrids or utility-scale grids), the problem will be resolved with different techniques [1]. Power System Resident Converter resident Hysteresis droop curve Microgrids Utility-Scale PV Frequency Regulation Methods Droop controller Local curtailment Utility-grid curtailment AGC individual curtailment AGC common curtailment Figure 1. Frequency regulation (FR) classification methods for photovoltaic (PV) applications. AGC: automatic gain controller. PV microgrids methods are mainly allocated inside power converters. There are two options: an open loop active power versus frequency control, with a hysteresis droop curve, or a defined look-up table; or a control droop method over the active power reference, in order to emulate the characteristic of synchronous generators against frequency excursions. All of them have been applied widely for PV microgrids [9,13–20]. However, these techniques are not able to be applied on USPVPs, because the capacity of one inverter could not be enough to move the grid frequency [21,22]. Notice that USPVPs solutions have two main groups. On the one hand, local regulation is done with a local active power curtailment, in a function of measured frequency and defined by the right grid code [2–7]. On the other hand, power system resident techniques are based on setting an external curtailment on the converters [1]. The curtailment value could come from the utility grid or from an automatic gain controller (AGC). The AGC could be a centralized or a distributed one, with the same control signal for every unit or with an individual control for each unit, respectively. Additionally, AGC methods could combine curtailment actions controlling deloaded reserves or energy storage systems (ESS) for further functionalities. MTRs for USPVPs describe power droop curtailment curves to help the system stability. Nevertheless, USPVPs cannot store by itself the remaining energy during a frequency disturbance. Consequently, FR techniques lead to production losses that must be considered. In strong grids, with a grid frequency value very stable, the problem is irrelevant because the occurrence of a fault during a year is very small [22,23]. However, in weak grids like Puerto Rico, Low California or the Sistema Interconectado Norte Grande (SING) in Chile, grid oscillations could vary significantly power production, and a comprehensive study of USPVP losses related to FR requirements is needed. USPVPs are built with a lot of PV electronic power converters. Nowadays, PV power converters generation response is very fast [24,25]. Therefore, they could reduce USPVPs impact over electric weak grids. Worldwide utility grids have modified MTRs with harder time response requirements, Energies 2016,9, 317 3 of 21 in order to use the advantage of the new power converter abilities. Reducing time response not only reduces the USPVP impact over grid frequency, but also helps the grid frequency stability. This clearly is always achieved at the expense of a loss of energy injected. Consequently, PV investors have to take this effect into account to calculate the plant profitability. The overall USPVP power output must remain according to MTRs, but additional systems could be installed to optimize the plant production. An ESS could be used to save the energy during fault events. Moreover, an ESS could be used to regulate other requirements such as ramp rates, curtailments, etc. [1]. The most popular technology for PV applications is a battery energy storage system (BESS) [13–23,26–29]. However, BESS prices are too high. Therefore, legislation of California, New York, Hawaii and Germany amongst others, provide BESS economic incentives [30–33]. The objective is to help the installation of new BESS, that otherwise would not be profitable. This paper shows some real cases where the installation of a BESS is justified without any need for economic incentives, and reduces the installation price kWh. This manuscript is organized as follows: Section 2 gives a review of international MTRs about FR. Section 3 describes the electric grid frequency behaviour, PV modelling, and expected power generation. Section 4 provides experimental test results about expected generation losses. Section 5 provides possible performance scenarios by introducing an ESS. Finally, discussions are given in Section 6. 2. Frequency Regulation 2.1. Grid Code Requirements Despite grid code harmonization being desirable, local grid conditions lead to local grid requirements defined for every country. Table 1shows recently published frequency ride through (FRT) and FR requirements [2–7]. On the one hand, FRTs set some thresholds to limit the converters frequency operation range. If the frequency reaches the limit after a defined time, the unit will be disconnected to prevent damage to the equipment. On the other hand, FR curves are pointed out. So, the power will be reduced applying a defined droop imposed by each country legislation. German and Italian legislations describe the classical droop curve. FR techniques apply when a starting frequency is passed (50.2 Hz for German code). Then a droop factor reduces the active output power of the plant with respect to the active output power previous to the beginning of the fault condition (PM). However, if the frequency is partially restored, the droop factor continues on his lowest value. The droop factor is cancelled only when the frequency recovers to nearly the nominal grid frequency value (50.05 Hz for German code). Other grid codes such as Mexico and Puerto Rico are not related to PM. Instead of that, they define an increment related to the nominal power plant capacity ∆P/Pn. Puerto Rico, Mexico and South Africa requirements look alike for over-frequencies, but establish special requirements for under-frequencies. Hysteresis curves, and the nominal output power of the plant (Pn) instead of PMare the main differences for over-frequencies faults behaviour. However, for under-frequencies, a deload reserve or an ESS is required, in order to increase the maximum available power of the plant (Pava). South African frequencies are pointed out with labels from f1to f6. f2and f3point out the thresholds to begin the FR action for under-frequencies and over-frequencies, respectively. f1and f4point out the frequency magnitude to saturate the frequency droop curve to Pava or the minimum required (Pmin), for under-frequencies and over-frequencies, respectively. Finally, f6points out the recovery frequency threshold. Finally, Chilean code is similar to German legislation, but FR recovery is not abrupt. However, the restoration is performed with the same frequency ramp than downwards, only it is timed to a maximum of 0.2 p.u./min. Energies 2016,9, 317 4 of 21 Table 1. FR utility-scale photovoltaic plants (USPVPs) minimum technical requirements (MTRs) review. FRT: frequency ride through; BDEW: German association of energy and water industries; CEI: Electrotechnical Italian Committee; NERSA: National energy regulator of South Africa; CFE: Comisión Federal de Electricidad; PREPA: Puerto Rico Electric Power Authority; CNE: National Commission of Energy. Country FRT FR Germany (BDEW) T (s) f (Hz) 0.0 50.0 47.5 60.0 51.5 1.0 PM f (Hz) 1.0 0.6 50.0 50.05 50.2 51.2 Italy (CEI) T (s) f (Hz) 0.0 50.0 47.5 60.0 51.5 1.0 PM f (Hz) 1.0 0.0 50.0 50.3 51.5 South Africa (NERSA) T (s) f (Hz) 0.0 50.0 47.5 47.0 51.0 52.0 60.0 48.0 49.0 51.5 0.2 10.04.0 6.0 46.0 PM f (Hz) Pava f1 50.0 52.0 f2 f3 f5 Pmin f6 Mexico (CFE) T (s) f (Hz) 0.0 60.0 57.5 62.0 600.0 58.0 58.8 62.5 5.0 61.2 0.1 ΔP/ Pn f (Hz) 0.1 49.75 50.0 52.5 -1.0 Puerto Rico (PREPA) T (s) f (Hz) 0.0 60.0 57.5 56.5 61.5 62.5 10.0 30.0 ΔP/ Pn f (Hz) 0.1 59.7 60.0 59.988 60.3 60.012 -0.1 Chile (CNE) T (s) f (Hz) 0.0 50.0 47.5 47.0 52.0 150.0 48.0 49.0 51.5 15.0 PM f (Hz) 1.0 0.0 50.0 50.2 f1 Energies 2016,9, 317 5 of 21 2.2. Frequency Regulation Methods Figure 2shows a typical PV converter control block diagram. The control scheme is divided into four layers: high, middle, low and hardware layer. FR capability for USPVPs is managed inside the middle layer, providing a suitable input data for the active power reference saturation threshold calculation. Typically, it could be performed locally or remotely. Remote active power saturation is set by Psatcurt signal, a power reference curtailment sent by the AGC. Local active power saturation is calculated inside the FR method block. Psatcurt Prefsp Qrefsp Pref Limits FR Method Pmax Pmin Pref´ Pref Idref Middle Level High Level Hardware Level Low Level Irst Vrst Vdc rst/dq0 Current Controller Duty Control Idq IGBT Drive System Middle Level Qmax Qmin Qref´ Pref Qref Limits Iqref Figure 2. PV converter controller scheme divided into 4 layers: high, middle, low and hardware level. FR method is allocated into the middle layer adjusting the output power reference output. The converter resident method acts in function of the local data of the converter, providing a droop factor over the maximum power saturation value. The droop factor (K) could be referenced to the nominal converter power [6,7] (Pn), or related to the active reference power previous to the frequency excursion [2–5] (PM). Figure 3points out a typical block diagram control of the FR control scheme. Maximum power point tracker (MPPT) output reference (Pref’) is modified by FR algorithm to provide a suitable current output power reference (Pref). AC DC MPPT xFR Method KPref ffn f z−1 PM Pref´ Figure 3. Converter resident FR scheme, maximum power point tracker (MPPT) output (Pref’) is modified by FR algorithm to provide a suitable current output power reference (Pref). The power system resident technique works in a similar way. However, every converter receives a curtailment saturation reference from an AGC (Psatcurt), who calculates the right power saturation Energies 2016,9, 317 6 of 21 value. The curtailment signal is due to many factors, one of them, the FR control of the PPC (Figure 4). Each inverter has a different output power level but all of them have the same curtailment signal. As a result, each inverter could be saturated in a different way. For example, at Figure 4, unit 1 has decreased its power (P(1))more than unit 2 power (P(2)), and unit 3 does not need to saturate its output (P(3)). But the power plant result will be the same that applying a converter resident solution. MPPT Pref’ (1) P(n) P(1) P(3) P(2) P, f AGC Psatcurt Pref (1) MPPT Pref’ (2) Pref (2) MPPT Pref’ (3) Pref (3) MPPT Pref’ (n) Pref (n) Psatcurt Pref’ (1) Psatcurt Pref’ (2) Psatcurt Pref’ (3) DC AC DC AC DC AC DC AC Figure 4. Power system resident FR by an AGC. A common control curtailment signal (Psatcurt ) is sent to all inverters. 2.3. Battery Energy Storage System Rule-Based Control The BESS should operate according to a rule-based control in order to achieve its objective. Figure 5shows the control law implemented in the BESS to get the results provided in this paper. The main objective of the BESS is to fulfil the mandatory FR MTRs. PFR PmaxBESS -PmaxBESS True False + + - SOCmax SOCmin SOC + + - PBESS RBESS Rp X+ + - SOCINI SOC-1 1 -1 ≠ 0 ηBESS /Ts ηBE SS ∫ Figure 5. Battery energy storage system (BESS) control law. BESS power output (PBESS) tries to provide the necessary FR active power contribution (PFR). If there was not a required FR action, the BESS would recover its initial state of charge (SOCini). The BESS power output (PBESS) tries to provide the necessary FR active power contribution (PFR), in order to minimize the USPVP losses. Therefore, there will be losses due to FR actions only when Energies 2016,9, 317 7 of 21 PFR is greater than the maximum active power capacity of the BESS (PmaxBESS), or when the state of charge (SOC) of the BESS system exceeds the range of operation defined ([SOCmin,SOCmax]). The efficiency of the BESS (ηBESS) has been considered to estimate the SOC correctly too. If no FR action was required (PFR is zero), then a recovery process could be made. The objective is to charge or discharge the BESS to its initial state (SOCini). Therefore, the BESS could regain its full capacity to compensate any sub-frequency or over-frequency occurrence. Moreover, a slope is introduced in the PBESS reference calculation (RBESS) to smooth the power output behaviour to the nominal power charging value (Rp). Finally, Tspoints out the sample time of the algorithm. 3. Frequency Regulation Behaviour Methods in Weak Grids 3.1. Weak Grids Frequency Excursions A classical approach divides power grids into two parts: transmission lines and distribution lines [34]. On the one hand, transmission lines connect huge power production plants with consumption centres via high voltage links. On the other hand, distribution lines go to households and industry with medium and low voltage lines. Weak grids have a relative small number of power sources and loads compare to a conventional national electric grid, so any unexpected change could produce voltage and frequency excursions. Consequently, these frequency excursions will lead the PV system to reduce its output production to comply with current MTRs. In order to quantize the problem, frequency excursions have been analysed for three different weak power grids: Puerto Rico (Puerto Rico Electric Power Authority (PREPA)), Chile (SING), and Low California or Mexico (Comisión Federal de Electricidad (CFE)). At least 20 days have been analysed for every grid to get reliable results. A grid power analyser set in a PV power plant was used to record the frequency measurements in Puerto Rico and Low California. However, SING frequency data measurements were provided by the utility grid. For all cases, the time sample of the measurements was 4s. Figure 6shows the time percentage in with a certain frequency value is over passed for these systems. In each graph, some key values are pointed out: the nominal frequency (fn); frequency threshold where the FR begins, flfor sub-frequencies and fhfor over-frequencies; and frequency threshold where the FR ends, fsl for sub-frequencies and fsh for over-frequencies. −0.5 fls fl fh fhs 0.5 10−3 10−2 10−1 (a) P(f) (p.u.) Puerto Rico −0.5 fn fh 0.5 10−3 10−2 10−1 (b) P(f) (p.u.) SING −0.5 fls fn 0.5 10−3 10−2 10−1 (c) f (Hz) P(f) (p.u.) Mexico Figure 6. Occurrence probability of a frequency excursion in three different power grids: (a) Puerto Rico (PREPA); (b) Chile (SING); and (c) Mexico (CFE). Energies 2016,9, 317 8 of 21 Note that every grid has different behaviours. The Puerto Rico probability disturbance spectra is almost symmetrical, SING has more over-frequency faults because there is over generation, and Low California has more sub-frequency faults because there is over consumption. In all cases, fland fh thresholds are over-passed with an occurrence greater than 10%, so production losses are significant and an analysis is justified. Moreover, Figure 7shows how long the frequency excursions are. Tmax points out the longest clearing time registered for a certain frequency disturbance. For every grid, results are of the same magnitude, registering faults near 50 min duration at fh. −0.5 fls fl fh fhs 0.5 25 50 75 100 (a) Tmax(f) (min) Puerto Rico −0.5 fn fh 0.5 50 100 150 (b) Tmax(f) (min) SING −0.5 fls fn 0.5 100 200 (c) f (Hz) Tmax(f) (min) Mexico Figure 7. Maximum clearing time of frequency excursions in three different power grids: (a) Puerto Rico (PREPA); (b) Chile (SING); and (c) Mexico (CFE). Figure 8points out the mean clearing time Tmn for such a disturbance. Note that the mean time of excursions after passing saturation thresholds (fhs and fls) goes up lightly. This is because the saturation of the FR droop control minimizes the restoration counter-effect of the method, incrementing the mean time of the perturbation. −0.5 fls fl fh fhs 0.5 1 2 3 (a) Tmn(f) (min) Puerto Rico −0.5 fn fh 0.5 1 2 3 (b) Tmn(f) (min) SING −0.5 fls fn 0.5 1 2 3 (c) f (Hz) Tmn(f) (min) Mexico Figure 8. Mean clearing time of frequency excursions in three different power grids: (a) Puerto Rico (PREPA); (b) Chile (SING); and (c) Mexico (CFE). Energies 2016,9, 317 9 of 21 Finally, Figures 9and 10 show the occurrence probability difference between day and night in the case of SING grid (Chile). Figure 9shows a greater faulty condition at day-time than at night-time, so FR techniques over PV plants are plenty justified. Figure 10 shows more detail for the frequency value fhwith the hourly frequency disturbance occurrence probability. Shadowed parts are related to night time. Three relevant peaks are presented, and all three are in day-time. −0.5 fn fh 0.5 0.0 0.1 0.2 0.3 0.4 f (Hz) P(f) (p.u.) Night Day Figure 9. Occurrence probability during the day and night of a frequency disturbance in SING power grid. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 0.0 0.05 0.1 0.15 0.20 0.25 P(f) (p.u.) Hour (h) Figure 10. Hourly frequency disturbance occurrence probability of fhin SING power grid. Shadowed parts point out night time. Therefore, Figure 6shows frequency behaviour for three remarked different types of weak grids. All cases show a high occurrence probability of frequency excursions compared to FR MTR thresholds, so an important decrease on PV power output is expected. Moreover, Figures 7and 8 point out significant frequency faults duration, so energy losses required to comply with MTRs will be very high and must be considered in the USPVP viability study. 3.2. Utility-Scale PhotoVoltaic Plants Sizing The availability of solar power sources must be considered. Production of a PV plant could be calculated from several environment measures. For example, a good approximation could be made with irradiance and temperature probes. The left side of Figure 11 shows typical irradiance curves of a solar plant in different weather seasons in the region of SING (Chile), and right side shows temperature measurements accordingly. The expected irradiance varies during the day. Moreover, the irradiance curve varies from summer to winter. Energies 2016,9, 317 16 of 21 Figures 20 and 21 show both expected losses and end of life (EOL) of BESS for Chilean and Puerto Rican cases. Above a certain value, increasing BESS capacity does not significantly reduce the energy losses of the whole system. However, the life expectancy could grow for several years, adding a little more capacity than the minimum required. 0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02 0.000 0.025 0.050 0.075 0.100 0.125 (a) eloss (p.u.h/day) C1 C2 C3 0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02 0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 (b) C (p.u.h) EOL (years) C1 C2 C3 Figure 20. BESS sizing results for SING case considering a whole year of transient results data, values of f1at 52, 51 and 50.5 Hz, C1,C2and C3respectively: (a) Daily energy losses (eloss) per BESS capacity; and (b) end of life (EOL) per BESS capacity. 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.02 0.03 0.04 0.05 eloss (p.u.h/day) (a) 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0 10 20 30 40 50 EOL (years) (b) C (p.u.h) Figure 21. BESS sizing results for Puerto Rican case considering a whole year of transient results data: (a) Daily energy losses per BESS capacity; and (b) EOL per BESS capacity. A typical value of EOL of a USPVP could be around 20 years. For example, in the Puerto Rican case, a capacity value of 0.025 p.u.h allows us to reach an EOL of 20 years. Figure 21 shows that the losses curve around C= 0.025 p.u.h is almost flat, so the selected Cvalue could be a suitable one. 5.3. Levelized Cost of Electricity Estimation LCOE is estimated for both cases: Puerto Rican and Chilean (f1= 50.5 Hz) plants. Two scenarios have been considered, with and without BESS, in order to quantize the difference between the two solutions. Energies 2016,9, 317 17 of 21 Parameters have been selected according to previous sections results and for a typical BESS and PV plant [46]. The weighted average cost of capital (WACC) have been set to 9.4% according to Table 5. BESS and USPVP settings are detailed in Table 6. Table 5. Weighted average cost of capital (WACC) settings. Parameter Value Parameter Value Debt 30.00% Debt interest 8.00% Tax equity 40.00% Tax equity interest 8.50% Equity 30.00% Equity interest 12.00% - - WACC 9.40% Table 6. USPVP and BESS settings. Description Case Chile Puerto Rico BESS Technology Li-ion Li-ion Nominal power 30 MW 30 MW Capacity 0.15 hp.u. 0.25 hp.u. Capital cost 1.5 $/Wh 1.5 $/Wh Efficency cycle 90% 90% Facility life 20 years 20 years O&M costs 165,000 $/year 165,000 $/year USPVP Technology Polycristaline Polycristaline Nominal power 300 MW 300 MW Capital cost 0.86 $/Wh 0.86 $/Wh Facility life 20 years 20 years O&M costs 6,000,000 $/year 6,000,000 $/year Finally, USPVP CAPital EXpenditures (CAPEX), OPerating EXpenses (OPEX) and LCOE values are pointed out in Table 7. BESS solution is more expensive, investment and operation costs are increased. However, active power production is increased significantly in both scenarios, so LCOE is reduced up to almost 3%. Table 7. USPVP levelized cost of electricity (LCOE) results. OPEX: OPerating EXpenses, CAPEX: CAPital EXpenditures. Country Case Without BESS With BESS Chile Production 749 GWh/year 767 GWh/year OPEX 6,000,000$ 6, 099,000$ CAPEX 258, 000,000$ 264,750,000$ LCOE 46.83 $/MWh 46.82 $/MWh Puerto Rico Production 656 GWh/year 702 GWh/year OPEX 6,000,000$ 6, 165,000$ CAPEX 258, 000,000$ 269,250,000$ LCOE 53.50 $/MWh 51.99 $/MWh Energies 2016,9, 317 18 of 21 6. Conclusions A comprehensive review of current MTRs related to FR and FRT capabilities for main international grid codes have been presented. FR requirements impose restrictions over active power production to help the utility grid stabilize the grid frequency. Moreover, reserves are needed to help the grid under low frequency excursions, reducing the output power capability of the USPVP. Weak grids have special MTRs and more severe frequency excursions. An analysis of frequency fault depth and duration behaviour with a wide spectra of real data have been presented for several weak grids with remarkable differences. Losses due to FR of two weak grids have been analysed, showing a considerable reduction of power production. Frequency measurements analysis has improved the frequency fault behaviour understanding in weak grids. The main conclusions about frequency faults behaviour are: • Frequency excursions could remian for very long inside the range of FR, up to 50 min for some cases. •However, the mean time of frequency excursions is much shorter, about 1 or 2 min for all studied cases. •When the frequency fault exceeded the FR operating range, the fault duration could be significantly longer. This effect is due to the FR saturation control. •Frequency excursions are hourly time dependent. The ocurrence probability of faults is higher in day time, increasing USPVP losses. Moreover, it is distributed irregularly with some remarked peaks, probably due to peak demand hours. PV plants have many advantages: its energy is clean, solar energy is a locally available renewable resource, etc. However, they are more expensive than other alternatives. Worldwide governments encourage solar plants with several types of incentives, in order to maximize the PV plants profits versus other technologies [47–51]. BESS could help to PV plants to become more profitable in some special cases as pointed out above, and contribute to help the grid system stability. Finally, the introduction of BESS has been proven to help reduce power losses and maximize the efficiency of the power plant, removing PV reserves. The improvement has been quantized with the LCOE estimation, with a reduction up to 1.51 $/MWh. This result is very important, because most recent reports point out the necessity of BESS economic incentives due to the high prices of batteries [30–33]. PV plant owners or utilities could evaluate whether new USPVPs can have economically viable BESS or not. Figure 22 shows a summarised procedure for a correct evaluation. The procedure could be divided into four phases. The first phase collects the most important environmental variables from the new plant’s desired location. Solar irradiance, temperature, location and other input settings build a model to size the USPVP accordingly with the owner’s objectives. The second phase estimates solar production generation (PPV), and a study about the grid frequency stability helps to estimate losses due to FR of national grid code (PFR). After that, the third phase sizes the more convenient BESS system for the studied application, and total power production could be estimated with and without BESS. Finally, the fourth phase calculates investment costs and revenues, and a final decision could be made redarding the most cost-effective solution. Moreover, introduction of BESS has many other advantages: •BESS could improve other features as constant power ramp rates over production. •Energy saved could be used at any time when the utility grid needs it, complementing the uncertainty of renewable sources. Moreover, BESS could perform perform peak shaving actions and load management. •BESS could be used also as static voltage compensators (SVCs), adding reactive power capacity to the plant. •BESS could smooth PV plant output production over cloudy conditions. •BESS could be used to perform load shifting actions to help the grid stability. Energies 2016,9, 317 19 of 21 USPVP sizing Irradiance Study Grid frequency Study BESS sizing Grid code PFR estimating PPV estimating + +- LCOE without BESS OPEX + CAPEX OPEX + CAPEX PBESS estimating + ++ LCOE with BESS <=0 + +- USPVP USPVP + BESS Yes No Ph1 Ph2 Ph3 Ph4 Figure 22. Summarised procedure to evaluate BESS whether new USPVPs can have economically viable BESS or not. Four phases are clearly distinguished: Phase 1: USPVP sizing; Phase 2: solar production generation and FR losses estimation; Phase 3: BESS sizing and total power production estimation; and Phase 4: LCOE calculation and decision. Acknowledgments: This work was supported by GPTech Spain, the Electronic Engineering Department of the University of Seville (Spain), and the Electronic Engineering Department of the University Federico de Santa María of Valparaiso (Chile). The authors acknowledge financial support provided by AC3E (CONICYT/FB0008), and by SERC Chile (CONICYT/FONDAP/15110019). This project has also received funding from European Regional Development Fund and the Ministry of Economy and Competitiveness, State programme of Research, Development and Innovation aimed at the Challenges of Society, R&D&I project (2013), Spain, under grant ENE2013-45948-R. The authors would like to thank Mr. Francisco Guerrero de Leste and José Alberto Vite Frías, from GPTech, and Matthew Lave, from Sandia National Laboratories, for their help. Authors also thank to the National Renewable Energy Laboratory for providing irradiance data of St. Thomas Bovoni Station for this study. Author Contributions: Jesús Muñoz-Cruzado-Alba conceived, designed and performed the experiments. Christian A. Rojas, Samir Kouro and Eduardo Galván Díez were responsible for the guidance and a number of key suggestions. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results. References 1. Morjaria, M.; Anichkov, D.; Chadliev, V.; Soni, S. A grid-friendly plant: The role of utility-scale photovoltaic plants in grid stability and reliability. 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