CRITICAL TIMEFRAMES OF IMPORTANCE FOR PV FROM A UTILITY PERSPECTIVE          Obadiah Bartholomy               Yong Cai    ...
performance during this timeframe varies primarily due to        provides 1 km and half hour time resolution, however fort...
between 5 and 6 PM.Typically, SMUD meets the majority of its peak demandneeds with internal natural gas and hydro generati...
penetrations, and for what frequency will help utility        basis, suggesting voltage control on such a feeder couldplan...
perceived by the non-PV resources on SMUD’s system.                                                              The resul...
historically to coincide with highly predictable solar          ensure that the utility is best prepared for significantre...
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ASES WREF 2012 Critical Timeframe of importance for PV from a utility perspective


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ASES WREF 2012 Critical Timeframe of importance for PV from a utility perspective

  1. 1. CRITICAL TIMEFRAMES OF IMPORTANCE FOR PV FROM A UTILITY PERSPECTIVE Obadiah Bartholomy Yong Cai Pramod Krishnani Belectric Inc. Brad Dommer 8076 Central Avenue Sacramento Municipal Utility District Newark CA 94560 6201 S St Sacramento, CA 95817 Email: Email: Brad.Dommer@smud.orgABSTRACT loads occur during approximately 15% of the 4,200 hours that PV produces energy during year in Sacramento,As solar penetrations increase, several resource attributes primarily in the springtime with a second but lowerand timeframes are becoming of increasing importance to concentration during the fall. Within an hour, a few dozenutility and balancing authority operators. In particular, days a year appear to have high variability, potentiallyoutput and variability of PV systems during summer peak creating issues for distribution system operators withdemand periods, during daytime minimum load changes exceeding 50% of rated capacity of a multi-MWconditions in the spring and fall, and finally at very high system in a 1-minute period. Finally, over the longer term,penetrations, during periods of significant load ramps for periods of significant ramps of PV in the morning andthe utility. Using historical solar resource data and load evening are likely to present challenges for existing griddata, PV output is simulated for different configurations balancing resources during non-summer months.and capacity levels to evaluate its potential to impact gridoperations during these types of timeframes. In this 1. INTRODUCTIONstudy, historical SMUD hourly load data is correlated tovalidated modeled PV output to determine appropriate on- Anecdotal assessment of PV output and problems canpeak capacity values by orientation, minimum load – tend to receive much of the focus and may leave long-maximum PV output correlations and frequency, and lasting impressions within a utility. Single events thatdiurnal patterns of change on an hourly basis. cause problems for a utility may reinforce perceptions of the resource as unreliable and problematic. For mostThe assessment uses 9 years of SolarAnywhere(c) 10 km utilities, high penetrations of solar are relatively new andgrid hourly data correlated to SMUD system load data. up until recently, high resolution historical datasets wereMore recent sub-hourly assessments are done for these not readily available to begin assessing the frequency thatvalues using recently installed ground-based solar problematic conditions may occur. This paper will beginmonitoring to evaluate sub-hourly variability that may to use these relatively recently available datasets tosignificantly impact grid operations during low-load time answer questions related to timeframes that are importantperiods of interest. to utility operators during which PV performance is of heightened importance.The results show that for SMUD’s service territory, peakcontribution from solar PV can be consistently expected The timeframe of primary importance to utility systemto contribute between 35% and 60% of its peak rating operators generally is during the hot summer months fordepending on orientation during the typical peak hour summer peaking utilities, and in particular, for SMUD,ending 6PM. Periods of high PV output and low utility during hot afternoons between 2PM and 8PM. PV 1
  2. 2. performance during this timeframe varies primarily due to provides 1 km and half hour time resolution, however forthe steady pattern of a declining solar resource as the sun the purposes of this analysis those data were not used.sets to the West. Late afternoon clouds are possible, butgenerally also reduce peak demand, or occur on days that In addition to the SolarAnywhere© data, SMUD, workingare not likely to reach critical peaks. with NEO Virtus Engineering Inc. under a grant from the CPUC California Solar Initiative RD&D program haveThe second timeframe that has been considered to be of deployed 71 solar monitoring devices around the serviceimportance for operations have been days where utility territory providing 1 minute data with 5km spacing. Theload is low, and the solar resource is high and variable. network was deployed midway through 2011, so only dataThese timeframes can be challenging at the distribution for the fall were used to evaluate minimum load highlevel as the PV resource is most likely to back-feed the variability conditions. Both datasets are shown in Figuredistribution substation and/or create undesirable voltage 1, which provides a view of SMUD’s service territory.fluctuations on a distribution feeder. Further, as solarpenetrations increase significantly, it is possible that solarwill push up against minimum load levels of hydrosystems in the spring and nuclear and combined cycle gasunits kept online as efficient baseload or load followingresources. In these instances, certain resources could becurtailed, as has been the case with oversupply of windand hydro resources in multiple locations around the USand the world1, or alternatively pricing in the marketcould go to zero or even negative, as has been explored byBaldrick for the ERCOT electricity market in Texas in2008 and 20092.Finally, resources for load following may be challengedduring times of significant slightly non-coincident diurnalramps between solar and the grid. Likely situations hereinclude slightly time delayed up and down ramps of the Fig. 1: Map of SMUD Service Territory,solar resource relative to morning and evening utility SolarAnywhere© 10 kM Grid blocks (blue) and NDFDramps. For timeframes where both ramps are steep, Grid cells and primary and secondary ground monitoringsignificant load following resources may be needed for sitesvery short periods of time, implying increased operatingcosts for these resources if it increases the starts and stops Solar data was modeled using the solar resource dataon thermal units in particular. These conditions, as well as based on a validated Excel based model for PV outputthe potential curtailment conditions are not likely to be that uses inverter efficiency, module temperature effects,problematic for a number of years until solar penetrations tilt, azimuth, and dust factors to estimate hourly solarare 6 or 7 times greater than current levels, however given output based on measured Global Horizontal Irradiancethe rapid growth of solar PV, it is prudent to begin (GHI) and Direct Normal Irradiance (DNI).exploring the conditions and planning for them today. 3. PV PERFORMANCE DURING PEAK DEMANDBy using 9 years of hourly 10 km resolution PERIODSSolarAnywhere© data, this paper examines PV output andcorrelated utility load during these timeframes to evaluate SMUD’s peak demand historically occurs during thehow challenging each may be for SMUD to address. summer months, most typically in July, but occasionally during June and August. The peak is driven primarily by2. SOLAR RESOURCE, PV MODELING AND LOAD residential air conditioning, which contributes DATA approximately 30% of overall demand during peak conditions. Overall, SMUD’s historic peak demand wasTo perform the analysis in this study, 9 years of hourly set in 2006 at 3,299 MW. Most typically, this demanddata were downloaded from the SolarAnywhere© tool occurs later than the CAISO peak due to the heavilycovering the Sacramento region. The data collected had a residential component. As temperatures rise in the hot10 km resolution, which is the standard resolution offered afternoon, residents come home from work and turn onby the tool. Enhanced resolution is also available which their air conditioning as businesses are still continuing to light and cool buildings, resulting in a peak that occurs 2
  3. 3. between 5 and 6 PM.Typically, SMUD meets the majority of its peak demandneeds with internal natural gas and hydro generation, andimports power from the Northwest and CAISO markets tosupplement our internal generation. However, as PVpenetrations increase, the utility will look to it as aresource to be counted on during times of peak demand.Because backup resources are most expensive during peakdemand periods, it will be of significant financialimportance to understand the solar PV resource duringthese timeframes. Fig. 3: Comparison of Modeled Output Profiles for PV systems on SMUD’s Peak Setting Day, 2006By looking at historical loads and modeled PV outputcoinciding with those peak demand periods, a clear 4. MINIMUM LOAD PERIODS WITH COINCIDENTpicture of the relative reliability of the solar resource MAXIMUM PV OUTPUTduring peak demand periods can be attained. For thisanalysis, we examined days with peak demand above Beyond peak periods, utility operators and planners are2,800 MW, which is approximately 500 MW below the most concerned with the operation of both distributionall time peak. Over the 9 year period, there were only 142 systems and the overall transmission system during lowhours which exceeded 2,800 MW, or roughly 0.18% of demand periods that coincide with high PV outputthe hours. Of these hours, the solar resource followed a periods. For SMUD, generally these periods occur duringconsistent pattern during the peak hours, contributing spring and fall months, where there is limited need forbetween 35% and 40% of rated output for a South facing cooling and/or electric heating, resulting in daytime loadssystem during the Hour Ending 6 PM for all of the 37 that may be between 30 and 40% of the peak demandpeak load days examined. This peak load and modeled period. Similarly, at the distribution level, minimum loadgeneration, along with the curves for SMUD’s all time periods generally occur during these same months, thoughpeak day set in 2006 are shown in Figure 2. can vary a bit more due to the different types of customers that may be connected to a given distribution feeder. Compounding the challenges associated with these time periods at the transmission level are high variability days and the availability of the hydro system to serve as a regulating and balancing resource. It is not uncommon under certain springtime runoff conditions for the hydro system to be running at full output to avoid spill situations. During these times, the system is not able to provide the regulation services that it is able to provideFig. 2: South facing PV Output during top 0.18% of Load during other times of the year. It is not able to ramp up orHours ramp down as it is running at full output. During fall periods, the hydro system may also be constrained in itsOne way to increase the amount of PV capacity available ability to provide regulation services, in particular in itson peak is by orienting it towards the West or ability to reduce output in response to increased PVimplementing tracking PV systems. This can increase on- output. This constraint can occur when there is inadequatepeak capacity available from a given system up to 60-65% water in the reservoirs to support generation that could beas shown in Figure 3, as compared to the 35-40% ramped downwards. While historically these conditionsavailable from a South facing system. have not been problematic for SMUD operators, as the utility adds significant amounts of solar these conditions are likely to increase in significance. As PV penetrations increase, it will become increasingly likely that coincident high PV, hydro, and wind generation could create conditions conducive to curtailment of resources during minimum load conditions. Understanding when these conditions may occur, at what 3
  4. 4. penetrations, and for what frequency will help utility basis, suggesting voltage control on such a feeder couldplanners identify optimal strategies for resource be challenging on such variable days. As the data has onlydevelopment, curtailment, energy storage, and been collected for a 6 month period, it is uncertain whatcomplementary grid resources that ensure flexibility and the total number of high variability days will be yet, but isminimize overall costs. For this analysis, we examined the expected to be between 40 and 60 days per year based onfrequency of low-load, high-PV output conditions that the data collected so far.would likely create challenges at the transmission level.The data were filtered to identify load periods between 30and 40% capacity factor that matched PV output periodsexceeding 50% capacity factor. The results aresummarized in Table 1.TABLE 1: PV Output during periods of minimum load(30% - 40% of peak), average values over 9 year studyperiod PV Total % Hours % Energy Capacity Hours Exceeding PV Exceeding PV Factor CF CF 50% 636 15% 25% 60% 373 9% 16% 70% 149 4% 7% Fig. 4: Modeled 1 minute change in output for a 3 MW 80% 15 0% 1% PV system for 4 variable days in October, 2011 90% 1 0% 0% 5. PERIODS OF SIGNIFICANT RAMPSIn addition to concerns at the transmission level, The last period of significance to a utility is generallydistribution system planners and operators have expressed much more broad than either of the first two. It is also likely to be further into the future than either of the firstconcern about PV driving voltage and backfeeding two concerns. This period really reflects the up and downsubstations which is not consistent with their design ramps associated with the rising and falling sun and theconditions. As noted above, many of these conditions willbe specific to the load characteristics of a given feeder, typical up and down of a daily utility load. While thehowever generally the same time periods are likely to be trajectory of solar crudely approximates the trajectory ofof concern as air conditioning loads are at a minimum. utility load, in fact most days there is an offset between aRather than concern about specific resources and markets, utility demand ramp and the ramp up of solar generation.distribution system concerns relate to the second to Similarly, as the solar resource dwindles, utility demandsecond and minute to minute variability of PV systems on often drops several hours later. The result of this slighta specific feeder. As some feeders may feature relatively mismatch is that as penetrations increase, resourcesdense concentrations of PV systems, high variability in previously used to meet demand will be significantly turned down and potentially off. While from a greenhousetheir combined output is possible, and could lead to gas mitigation perspective this is ultimately the goal, forreliability concerns and substandard voltages for other utility operators this may mean significant increases incustomers on the feeder. SMUD is currently collectingdata from our high density solar monitoring network to operating costs of thermal units required to ramp up andsupport evaluations of PV output at the distribution level, back down, and potentially off, followed by a restart andand will be working with Clean Power Research to second ramp up during the evening. Adding one restartevaluate variability at the feeder level using high per day could substantially increase O&M costs andresolution SolarAnywhere© data. would undoubtedly impact plant emissions and efficiencies. Alternatively, it could mean the deploymentIn the meantime, analysis using SMUD’s high-density of significant storage resources, new targets for demandsolar monitoring network indicates that multi-MW response applications, and/or other zero emission flexiblesystems may cause significant challenges during high- generation as a means of addressing the issue without increasing emissions.variability days on a given feeder. SMUD currentlyallows PV to be sized to 100% of minimum daytime loadon a feeder. This data indicates changes that would be theequivalent of 50% of this load occurring on a 1 minute 4
  5. 5. perceived by the non-PV resources on SMUD’s system. The result would be two-fold. First, SMUD could be forced to either curtail PV generation or sell generation into what would likely be a low-priced electricity market, Minimum load as evidenced by what is occurring in Germany currently as a result of high PV penetrations3. Second, as SMUD ramped up generation to meet load in the morning, peaking or simple-cycle load following generation would be required to be turned on to cope with the short duration of need. This same type of generation would be looked to again as the sun set to replace PV in the resource mix and meet the evening load. Such operation would represent as significant departure from current utility operations, as it would rely much more heavily on simple cycle generation and would incur significantly increased O&M costs Minimum load associated with these types of generation, given current operations rarely require more than one start in a day, and generally involve operation for 6 to 8 hours rather than a 2 to 3 hour operating mode as indicated in Figure 6. The intent of this significantly simplified assessment is to point out the challenges in operations that could occur as a result of significant penetrations of PV and to begin to identify mitigation approaches for dealing with these challenges. It is likely that interaction with the broader Minimum load Northwest and California electricity markets may mitigate some of the concerns with regard to resources, and further that other resources on SMUD’s system, in particular our Upper American River System, would be used for meeting some of the ramping demands, alleviating much of the concern regarding double-starts of thermal units. However, as PV penetrations increase statewide, availability of market units and or hydro assets for early morning ramps may become increasingly unlikely as Minimum load these more flexible resources are deployed to deal with intermittency issues and firming of solar and wind. 6. CONCLUSIONS Utility operators have historically been distrustful of theFig. 5: Ramp timing implications for solar and load for ability of PV resources to meet their needs from aMarch, June, September and December, modeling 1,000 reliability perspective. Concerns about consistency onMW PV and average loads peak, low-load variability, and implications for complementary grid resources generally have hamstrungFor this assessment, as described in Figure 6, it was widespread acceptance of PV as a valued grid asset.assumed that the minimum load threshold in each night However, using tools that have only been recentlyrepresented the minimum turndown for utility resources. developed, utilities today are much more able to modelIt was also assumed that SMUD expanded its current output of significant penetrations over longer historicalinstalled PV capacity from an expected 140 MW at the periods to better understand how these generating unitsend of 2012 to 1,000 MW, which is currently beyond perform during time periods of interest.anything in the 20 year planning horizon, but is certainlypossible if PV prices continued to drop and carbon In this paper, three time periods of interest were exploredconstraints drove significant additional renewables that could pose challenges to the utility depending on thebeyond 2020. In this scenario, a new minimum load performance of PV generating units. The first time period,would be achieved during the daytime, at least as corresponding to times of peak demand, was found 5
  6. 6. historically to coincide with highly predictable solar ensure that the utility is best prepared for significantresources. In fact the primary variables related to increases in solar generation in our mix.predicting solar output on a peak day relate more to theorientation of the system than the availability of the solar (1) Porter, K., Rogers, J., Wiser, R. “Update on Windresource. Based on examination of 37 days representing Curtailment in Europe and North America” June 16, 2011the top 0.18% of peak demand hours, the modeled PV was found to perform within a very narrow 377.1820833464392.pdf/Update%20on%20Wind%20Curband between 35% and 40% capacity factor for a SouthFacing system during the peak hour. As a result, it wasdetermined that PV generation could be counted on for aconsistent amount of output during these time periods, (2) Baldrick, Ross “Wind and Energy Markets: A Casewhich will assist in planning for SMUD’s future Load Study of Texas” 2011, US Association for EnergyServing Capability with large penetrations of solar on the Economics Dialoguegrid. &view=article&id=109&Itemid=170The second timeframe of significance, the coincidence of Accessed 3/2/2012 3:03 PM PSTlow utility load and high PV output was also examinedfrom a macro and micro level. At a macro level, (3) Shahan, Zachary “Solar PV Reducing Price ofuneconomic conditions could occur leading to curtailment Electricity in Germany” February 9, 2012or sale of generation into markets at rates below the long- CleanTechnica.comterm expected value of that generation. The frequency of types of conditions could represent as much as 7% price-of-electricity-in-germany/ Accessed 3/2/2012 5:37of the hours of the year, where utility loads were between PM PST30 and 40% of peak and PV output was greater than 50%of rated capacity. In addition, at the distribution level,extreme variability, including ramps of up to 50% ofcapacity in a minute is likely to create issues on a fewdozen days of the year. These conditions could potentiallylead to local curtailment, changes in distribution systemdesign, or the inclusion of new distributed storage or othergrid assets to compensate for these extreme conditions.Finally, over the longer term, scenarios may developsimilar to those in Germany, where daytime loads net ofPV are as low or lower than nighttime loads. Withsignificant PV penetrations throughout the state, and thenon-coincidence between load and solar ramps, existinggeneration will be less useful in filling in short term needsfor morning and evening generation, leading to apotentially significantly price differentiated market.Simple cycle generation, storage, and demand responsewill likely be better suited for meeting ramps thancombined cycle generation given the short lengths andsignificant ramp magnitudes that may occur.Understanding these conditions can help guide the typesof grid resources that utilities plan for over the nextdecade.This exercise was an initial attempt to begin to answersome of the questions around the impacts of PV onexisting utility operations. Further work is expected torefine these assessments, and develop more realistic long-term scenarios against which to evaluate complementarygrid resources. However, the exercise has shown thebenefits of beginning to think about these timeframes to 6