THE VALUE OF GRID-SUPPORT PHOTOVOLTAICS IN PROVIDING DISTRIBUTION SYSTEM VOLTAGE SUPPORT 2. OBJECTIVE

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1 THE VALUE OF GRID-SUPPORT PHOTOVOLTAICS IN PROVIDING DISTRIBUTION SYSTEM VOLTAGE SUPPORT T. Hoff and H. J. Wenge Pacific Enegy Goup Walnut Ceek, Califonia USA B. K. Fame Pacific Gas and Electic Company San Ramon, Califonia USA ABSTRACT Stategically sited gid-suppot photovoltaic (PV) applications have been poposed to povide distibuted value (cost savings) to electic utilities expeiencing tansmission and distibution (T&D) system oveloads. These applications can potentially defe capital upgades, extend equipment maintenance intevals, educe electical line losses, and impove distibution system eliability. This eseach focuses on one aspect of the value of gid-suppot PV: the value to a substation tansfome load tap change. Results at Pacific Gas and Electic Company indicate that, due to the voltage suppot povided by the.5 MW PV plant at Keman, Califonia, the lifetime opeation and maintenance costs of a tansfome load tap change at the Keman Substation ae educed by $,. Although the method is geneally applicable, the esults ae site specific.. INTRODUCTION A common pactice of electic utilities expeiencing tansmission and distibution (T&D) system oveloads is to expand the substation, add lines, o upgade equipment, all of which ae capital intensive options. In 988, it was hypothesized that stategically sited photovoltaics (PV) could benefit pats of T&D systems nea o at oveloaded conditions []. An evaluation methodology was developed and applied to a test case (Keman Substation nea Fesno, Califonia). Results of this and othe studies suggested that the value of PV to the T&D system could exceed its enegy and geneation capacity value [, ]. The impotance of this finding indicated the need fo empiical validation. This led to the constuction of a.5 MW PV demonstation plant by Pacific Gas and Electic Company (PG&E) at Keman, Califonia as pat of the PVUSA (PV fo Utility Scale Applications) poject. PVUSA is a national coopeative eseach and development effot unde the auspices of the United States Depatment of Enegy []. PVUSA developed guidelines of how to configue the plant to obtain the geatest total value [4] and designed a eseach test plan [5] to empiically detemine the value of PV to the T&D and bulk geneation systems. The Keman PV plant, completed in June, 99, is epoted to be the wold s fist gid-suppot PV demonstation plant.. OBJECTIVE Gid-suppot PV can povide many values to T&D systems. It can defe capital upgades [6, 7], extend equipment maintenance intevals, educe electical line losses [8], and impove distibution system eliability, all with cost savings to utilities. This eseach examines the value of gidsuppot PV to a substation tansfome load tap change (LTC). Utilities stive to maintain cetain powe quality standads thoughout thei sevice teitoies. One way they accomplish this is though the use of voltage egulation devices such as LTCs. Gid-suppot PV may povide value to an LTC by extending maintenance intevals and thus educing maintenance costs. It achieves this by boosting voltage on the feede which displaces some of the voltage egulation povided by the LTC. This pape develops a method to quantify the level of feede voltage suppot povided by a PV plant, the effect of this on LTC pefomance, and the coesponding economic value. The method is then applied to the.5 MW PVUSA PV plant located nea PG&E's Keman Substation.. METHODOLOGY. Voltage Suppot Povided by PV Plant Voltage suppot (VS) povided by PV is the diffeence between voltage dop ( V) with and without the PV plant. Since voltage dop is the poduct of cuent and impedance, voltage dop without PV between a substation tansfome and some location on a feede equals V =I x 66 I l Z l dl whee cuent (I) and impedance (Z) ae functions of feede location (l), is the tansfome location, and x is the location of inteest on the feede. ()

2 Cuent With PV Voltage Without PV Equation (4) states that voltage suppot at any paticula feede location is the poduct of PV plant cuent and conducto impedance between the tansfome and the point at which the lateal is attached to the line between the tansfome and PV plant. The implications of this ae: ) voltage suppot is independent of feede cuent; ) voltage suppot is linealy elated to plant output (and thus to plant size); and ) voltage suppot anywhee on a feede is known and is based only on PV plant output and feede configuation. Substation Tansfome low esistance wie A B I PV high esistance wie C PV Geneation Fig.. Feede voltage suppot povided by PV. As illustated in Fig., a PV plant educes voltage dop by educing feede cuent. Voltage suppot povided by a PV plant to point B on the lateal in Fig. is the diffeence in voltage dop with and without PV. Voltage dop without PV is simply () with B substituted fo x. Voltage dop with PV equals IA B VB = I6 l IPV Z6 I l dl + I66 l Z l dl which, since I PV is constant ove the line at any instant in time, equals V = I l Z l dl I Z B IB 66 PV whee Z A is the total impedance fom the substation tansfome to point A. Substituting B fo x into () and subtacting (), voltage suppot at point B is A A () () VSB = IPVZ A. (4) A simila analysis will show that the voltage suppot at point D equals I PV Z C. Note that impedance in (4) can be eplaced with esistance when the PV opeates at unity powe facto. D. Paametes Affecting LTC Opeation Tanslating voltage suppot to economic value equies undestanding the elationship between LTC changes and feede voltage equiements. LTC opeation is based on voltage into the LTC, feede voltage egulation devices online, LTC voltage ange setting (this ange is set to compensate fo voltage dop on the feede), and the vaiation in tansfome load. This subsection outlines some of the most impotant paametes and how they elate to LTC changes. Fig. pesents the elationship between desied substation voltage and the LTC voltage ange setting and tansfome load. The figue illustates that desied substation voltage is a linea function of tansfome load given an LTC voltage ange setting. Fo illustation puposes, two settings ae consideed with the wide voltage ange being twice the naow voltage ange. The LTC voltage ange is set to naow when small voltage dops ae anticipated on the feede; the ange is set to wide when lage voltage dops ae anticipated. Many moe settings ae possible in pactice. Desied substation voltage as a function of load is conveted to LTC changes by combining Fig. with a load pofile, such as the one pesented in Fig.. The load axis is common to both figues so the esulting plot, Fig. 4, is desied substation voltage vesus time. The top dashed cuve coesponds to desied voltage fo a wide voltage ange and the bottom to a naow voltage ange. Actual substation voltage fo any given input voltage is limited by the numbe of available LTC taps and the voltage change poduced by one LTC tap change (LTC step size). Thus, thee is a diffeence between desied substation voltage (a continuous function) and actual substation voltage (a step function). When this diffeence exceeds the LTC bandwidth, the LTC changes taps.

3 Desied Substation Voltage Wide Range Naow Range Load Fig.. Voltage vesus load fo two voltage ange settings. Fig. 4 plots actual substation voltage in addition to desied substation voltage. LTC bandwidth and LTC step size ae the same fo both cases; only the LTC voltage ange setting is diffeent. The figue suggests that, in geneal, thee is a linea elationship between the numbe of LTC changes and voltage ange setting when all othe vaiables ae constant. Fo example, the fist fou LTC changes fo the wide voltage ange coespond to two LTC changes fo the naow ange. This is as expected since the naow ange was selected in Fig. to be half the wide ange. Notice, howeve, that although thee is an additional step up in the wide ange case, thee is no coesponding LTC change in the naow ange case. This is attibutable to the fact that the LTC does not have a continuous ange of voltages available. As the voltage ange setting naows, factional LTC changes ae eliminated. This is not impotant if actual loads follow the load patten pesented in Fig.. Load In eality, howeve, load fluctuations occu thoughout the day in addition to the oveall daily load change. An LTC change occus if these load fluctuations ae lage enough and the LTC voltage ange is sufficiently wide; they stop occuing as the LTC voltage ange naows. In ode to make the esults of this eseach boadly applicable, it is desiable to develop an equation that descibes LTC changes as a function of voltage ange setting. Following is one suggestion of what that equation might look like. Substation Voltage Time Fig.. Hypothetical load pofile. Wide Range Actual Desied Time Fig. 4. Desied and actual voltages. Actual Desied Naow Range The discussion in the pevious paagaphs suggests that one possible model is a combination of a linea and a non-linea equation whee some faction of the changes (F) ae associated with the linea equation and the othe (-F) with the non-linea equation. If one assumes that the magnitude of the load fluctuations thoughout the day have an exponential pobability distibution, one non-linea equation that makes sense is an exponential function and the combination of the two equations becomes C = C F meas! R LTC R LTC meas step step + Fe 6 SR R meas 6 whee C is the numbe of LTC changes fo an LTC voltage ange setting of R, C meas is the ecoded numbe of LTC changes coesponding to an LTC voltage ange setting of R meas, LTC step is the voltage change poduced by one tap change, and S is a scaling facto that detemines the slope " $ # (5)

4 of the exponential cuve. Equation (5) is valid as long as R is geate than o equal to LTC step. Given LTC step and a C meas associated with some R meas, F and S in (5) ae needed befoe C is a function of only one vaiable, R. F, a unitless vaiable that descibes what faction of changes is attibutable to the linea and nonlinea tems in (5), is estimated by 65 Lmax Lmin6 F = (6) C LTC meas step whee L max minus L min is the aveage daily load change (in MW/day), is the desied voltage change pe MW of load change (volts/mw), LTC step is the voltage change poduced by one tap change (volts/step), and C meas is the ecoded numbe of LTC changes in a yea (steps/yea). Thee ae 65 days pe yea and the accounts fo the fact that if the LTC setting inceases to meet the maximum load, it must decease to meet the minimum load. S, the scaling facto that detemines the slope of the exponential cuve, is estimated by inputting at least two sets of LTC changes unde two sets of conditions into (5) and solving fo S. Its value can be appoximated if two sets of histoical LTC data ae unavailable.. Economic Value The technical esults fom the pevious subsections can be tanslated to economic value by computing the diffeence between the LTC maintenance cost with and without the PV plant. Specifically, one calculates the net pesent value of the maintenance cost with the PV plant and subtacts this fom the net pesent value of the maintenance cost without the PV plant. The diffeence between the two is the value of PV to the tansfome LTC. The net pesent value (NPV) of a peiodically ecuing maintenance cost is a geometic seies that educes to + + NPV = C MI +! MI N+ 6MI SL + + N MI SL " (7) # $ whee C is the cuent LTC maintenance pocedue cost, is the ate of inflation, c is the cost of capital, MI is the maintenance inteval (it is expessed in yeas and equals the numbe of LTC changes between maintenance pocedues divided by the annual numbe of LTC changes), N is the numbe of times the maintenance pocedue is pefomed ove the study life, and SL is the study life. This expession is most familia fo annually ecuing expenses, in which case MI equals, (N+)MI equals SL+, and the second tem in the backets is zeo [9]. Equation (7) includes the second tem to account fo times when the study life is not an intege multiple of the maintenance inteval. In these cases, a factional maintenance pocedue must be pefomed at the end of the study life. 4. RESULTS This section applies the method fom the pevious section to the.5 MW PV plant located nea PG&E s Keman Substation. The.5 MW plant is located on Feede, one of the two feedes egulated by the Keman Bank tansfome LTC. Results, which ae site specific, ae based on measued data fom this plant. 4. Voltage Suppot Povided by PV Plant Field tests of Keman Feede wee pefomed on July, 99 and July 6, 99 that facilitate quantification of the voltage suppot povided by the.5 MW PV plant. On July 6, a ange of feede load conditions, including exteme peak load conditions (6 MW, o pecent geate than nomal peak loads), wee simulated by having the distibution system opeato tansfe load fom adjacent feedes to the Keman Feede. Fig. 5 pesents the voltage suppot povided by the PV plant at the plant location as a function of plant cuent and feede load. The figue suggests that voltage suppot is linealy elated to PV plant cuent and is independent of feede load. The Model line is constucted by multiplying plant cuent times feede esistance as descibed in (4). The PV plant boosts feede voltage at the plant by moe than.5 volts at full plant output. In addition to automatic measuements taken at the PV plant, feede voltage was measued manually at thee citical feede locations. The distibution system opeato took the PV plant off-line, field pesonnel measued thee phase voltage, the distibution system opeato put the plant back on-line, and field pesonnel emeasued the voltage. It took about 5 minutes between the fist and last voltage measuements. Feede cuent was not monitoed at each location; athe, it was monitoed evey minute on only one phase at a location othe than whee the voltage measuements wee taken. 4

5 Fig. 6 pesents the voltage suppot povided by the PV plant as a function of the distance along the line between the PV plant and the tansfome (i.e., the distance of the lateal is not included). The single line diagam in the figue is dawn to scale and shows whee the voltage measuements wee taken (they ae maked with a box; manual measuements ae maked with an M). Modeled values wee calculated based on a plant output cuent of Amps (.4 MW), feede esistance, and (4). Although not a pefect elationship, the figue suggests that voltage suppot is independent of feede load and that it is the point at which the lateal is connected to the line between the tansfome and PV plant athe than the distance fom the tansfome that detemines the level of voltage suppot. The less than ideal esults may be explained by the long time delay (5 minutes) between measuements, the lack of cuent measuements at the locations of inteest, and that the measuement instuments wee disconnected between measuements. The esults in Figs. 5 and 6 ae tanslated to feede voltage suppot by examining voltage suppot at the feede location with the lowest voltage; this location may change as PV plant size inceases. Results ae tanslated to voltage suppot on a goup of feedes (and thus to the LTC since it seves two feedes) by examining minimum voltage on all feedes seved. Minimum voltage location on adjacent feedes, howeve, does not change since PV plant voltage suppot to adjacent feedes is zeo. Fig. 5 implies that, since voltage suppot is linealy elated to PV plant output, it is also linealy elated to PV plant size fo plants opeating at a given pecentage of thei ating. Thus, calculating minimum voltages with the PV plant online equies knowing what the voltages wee with the PV plant off-line and the voltage inceases pe unit of PV. Minimum voltages with the PV plant off-line duing the July 6, 99 test wee 8.9 volts, 6.6 volts, and 8.5 volts at the manual measuement locations M, M, and M in Fig. 6. Fig. 7 pesents minimum voltage on the goup of feedes that the LTC seves as a function of PV plant size (dak solid line) by combining these initial voltages with the voltage suppot povided by a PV plant opeating at 8 pecent of its ating; minimum feede voltage on an adjacent feede (labeled adjacent feede) is assumed to be.5 volts. The figue suggests that a plant ated at.5 MW boosts voltage by about volts at the location with the lowest minimum voltage. Voltage Suppot (volts) Feede Load - MW 4-5 MW -4 MW 5-6 MW Model PV Plant Output (MW) Voltage Suppot (volts) Substation Tansfome Feede Load. MW 4.4 MW. MW 4.7 MW.5 MW 5.9 MW Model Distance (miles) PV Plant 75 Al 4/ Al #6 Cu #6 Cu #6 Cu M Fig. 5. Voltage suppot vesus PV plant output at vaious feede load levels (July, 99 and July 6, 99). M M Fig. 6. Voltage suppot povided by PV plant at.4 MW output at vaious locations (July 6, 99). 5

6 Voltage ( volt base) M Adjacent Feede M Minimum Voltage M Annual LTC Changes ('s) 5 4 Model Measued Model Data C 9 C 9 C 9 C Plant Size (MW) Fig. 7. Voltage suppot vesus PV plant size. 4. LTC Changes and LTC Voltage Range Setting The elationship between LTC changes and voltage suppot must be undestood to tanslate PV voltage suppot to a eduction in LTC changes. The squaes in Fig. 8 epesent the measued numbe of LTC changes fo 99 though 99. All points epesent a yea's woth of data except 99, which is based on 4 pecent of the yea and scaled to an annual estimate. Fig. 8 includes model esults. The data used to constuct the model ae denoted by the cicles. As descibed in (5), the model is based on the ecoded numbe of LTC changes (C meas ) coesponding to a given LTC voltage ange setting (R meas ), the voltage change poduced by one tap change (LTC step ), the faction of load changes associated with daily load changes (F), and the scaling facto that detemines the slope of the exponential cuve (S). Although the LTC bandwidth changed slightly between yeas (±.75 volts in 99 and 99, and ±.65 volts in 99 and 99), this change is ignoed. C meas associated with an R meas of 5. volts equals 4,4 LTC changes pe yea (i.e., the aveage of LTC changes in 99 and 99); LTC step equals 9/6 volt; F equals.44, and is calculated using (6) and estimates of the aveage daily load change (L max -L min ) of.6 MW/day and of.5 volt/mw (i.e., voltage ange of 5 volts divided by maximum tansfome load of MW); and S equals.4, and is detemined by solving fo S in (5) and then inputting 99 and 99 conditions. The figue suggests that the model is a good fit to measued data. 4 5 LTC Voltage Range (Volts) Fig. 8. Annual LTC changes vesus voltage ange setting. 4. Economic Value Economic value can be calculated now that the voltage suppot povided by the PV plant and the elationship between LTC voltage ange setting and annual LTC changes ae known. As descibed ealie, the economic value of a PV plant to the LTC is the diffeence between the LTC maintenance cost without the PV plant and the maintenance cost with the PV plant. The LTC is outinely inspected evey fou to six yeas. Duing the inspection, won pats ae eplaced. At the Keman Substation, it is estimated that the vaiable maintenance cost (C) associated with pats eplacement is $5, and occus evey 5, LTC changes. Accoding to Fig. 8, the cuent LTC setting coesponds to 4,4 changes pe yea. This tanslates to a maintenance inteval (MI) of.4 yeas. Using (7) and the assumptions that inflation () is 5. pecent, cost of capital (c) is. pecent, and the study life (SL) equals the PV plant life of yeas, the net pesent value of the LTC maintenance cost ove yeas without PV is $6,4. Accoding to Fig. 8, a new LTC voltage ange setting of.5 volts coesponds to about, changes pe yea. This tanslates to a maintenance inteval of about 5 yeas. Using the same assumptions as above and (7), the net pesent value of LTC maintenance cost ove yeas with a.5 MW PV plant is about $,4. Thus, the value of PV to the LTC is the diffeence between the oiginal cost ($6,4) and the new cost ($,4) o $,. Fig. 9 pesents this same calculation fo a ange of PV plant sizes. 6

7 Net Pesent Value $, $5, $, $5, $ Plant Size (MW) Fig. 9. LTC value vesus PV plant size. 5. CONCLUSIONS AND FUTURE RESEARCH A simple method was developed in this pape to estimate the value of gid-suppot PV to a substation tansfome LTC. The hypothesis was that PV educes LTC maintenance costs because LTC changes ae a function of feede voltage suppot equiements and a PV plant educes these equiements by poviding feede voltage suppot. This voltage suppot tanslates to a educed LTC voltage ange setting. Results suggest that the.5 MW PV plant nea the Keman Substation will save $, (NPV) in LTC maintenance costs ove the yea life of the plant. Impotant obsevations esulting fom this wok ae: ) voltage suppot povided by a PV plant (o any othe fom of distibuted geneation) is independent of feede cuent; ) voltage suppot is linealy elated to plant output (and thus to plant size); and ) voltage suppot anywhee on a feede is known and is based only on plant output and feede configuation. Futue eseach needs can be divided into economic and technical categoies. Fom an economic pespective, the possible pevention of LTC failue, and thus the defement of a capital expenditue, has not been consideed. This value might exceed the maintenance savings estimated in this pape. In addition, moe eseach is needed to assess the elationship between the numbe of LTC changes and maintenance costs. An examination of histoical cost data would be beneficial. Fom a technical pespective, the model elating the LTC voltage ange setting to LTC changes needs futhe evaluation. A specific aea of concen is the omission of tansmission voltage in the model. 6. ACKNOWLEDGMENTS It was though the effots of PG&E and othe PVUSA membes that the.5 MW Keman PV plant was constucted so this analysis could be pefomed. Joaquin Buendia of PG&E's Fesno Division pompted initial fomulation of the hypothesis and demonstated unending patience in answeing all the questions necessay to pefom the analysis. Valuable comments wee eceived fom Ken Lau, Jimmie Yee, John Monastaio, and John Matin of PG&E, and Geoge Tennyson of the Depatment of Enegy. Sam Dooms of Utility Distibution Electic obtained data cucial to this analysis. Jim Augustyn and Rob Nelson of Augustyn + Company pefomed special data collection tests. John Weyant of Stanfod Univesity povided enthusiastic suppot and inteest in this wok. 7. REFERENCES [] D. S. Shuga, "Photovoltaics in the Utility Distibution System: The Evaluation of System and Distibuted Benefits," Poceedings of the st IEEE PV Specialists Confeence, Kissimmee, Floida, May 99. [] R. Lambeth and T. Lepley, "Distibuted Photovoltaic System Evaluation by Aizona Public Sevice Company," Poceedings of the d IEEE PV Specialists Confeence, Louisville, Kentucky, May 99. [] T. R. Candelaio and T. Townsend, "PVUSA - Pogess and Plans," Sola 9, 99. [4] T. Hoff, D.S. Shuga, and H.J. Wenge, "Economic Detemination of Optimal Plant Design fo Photovoltaics in the Utility Distibution System," Poceedings of the nd IEEE PV Specialists Confeence, Las Vegas, Nevada, Octobe 99. [5] D. S. Shuga and T. Hoff, "Gid-Suppot Photovoltaics: Evaluation of Citeia and Methods to Empiically Assess Local and System Benefits to Electic Utilities," Pogess in Photovoltaics, Reseach and Applications, John Wiley and Sons, July 99. [6] T. Hoff and D.S. Shuga, "The Value of Gid-Suppot Photovoltaics to Substation Tansfomes," Submitted fo the 994 IEEE/PES Summe Meeting, San Fancisco, Califonia, July 994. [7] M.M. El-Gassei, K.P. Altenede, and J. Bigge, "Enhancing Tansfome Dynamic Rating Though Gid Application of Photovoltaic Aays," Poceedings of the d IEEE PV Specialists Confeence, Louisville, Kentucky, May 99. [8] T. Hoff and D.S. Shuga, "The Value of Gid-Suppot Photovoltaics in Reducing Distibution System Losses," Submitted fo the 994 IEEE/PES Summe Meeting, San Fancisco, Califonia, July 994. [9] R.A. Bealey and S.C. Myes, Pinciples of Copoate Finance, McGaw-Hill, 99, pp