Applying Capacitors for Peak Shaving.pdf

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Spring 2002 1

As utility companies move into a competitive marketplace, they are

more mindful than ever of customer service and cost savings. As

such, peak load reduction is more important today than ever

before. There are many locations where the power generated is approxi-

mately equal to load. If load is not reduced, customers may receive volt-

age below the stated minimum levels or may face extended outages be-

cause the utility must curtail load to remain stable. The normal price of

energy on the open market varies between $30 to $50 per MWh. In recent

summer peaks, prices have soared to $7500 per MWh. By reducing peak

demand, a utility can avoid paying premium values when purchasingpower or may sell excess generation at a premium price. Utilities caught

short in these peak times have suffered a substantial financial penalty.

Conservation Voltage Reduction (CVR)CVR is the process of lowering the substation voltage to reduce de-

mand during peak loading conditions. Utilities have been aware of the benefits of CVR for many years. While test results vary from circuit tocircuit and from utility to utility, results usually indicate the load is de-creased by approximately 1.0 percent for each 1.0 percent drop in sub-station voltage. Results from the most recent tests by Georgia Power de-

termined a 0.8 percent load reduction for each one volt decrease, using a120 volt basis.

For CVR to be effective, the feeder loads must have a significant com-ponent of constant impedance loads – loads that have a predominantlyresistive characteristic. The most common constant impedance load islighting. As the voltage supplied to an incandescent light bulb is de-creased, the current through the filament decreases. Therefore, the en-ergy used by the light bulb is reduced. While the effect of a single light bulb is negligible, a feeder serving several megawatts of lighting loadpresents an opportunity for peak shaving.

by Christopher McCarthy

Cooper Poswer Systems

An obvious requirement for be-ing able to perform voltage reduc-tion on a feeder is that the feedermust have a voltage profile suffi-ciently above the minimum accept-able voltage delivery level. On a120-volt scale, the common upperand lower limits of the voltage pro-file are plus or minus five percent,or 114 to 126 volts. If the voltage atthe end of the line is only slightlyabove the 114 volt threshold dur-ing peak conditions, then furtherlowering the voltage to reduce de-mand is not practical.

Georgia Power has developed amethod to make CVR work onmany of their heavily loaded cir-cuits that suffer from a significantvoltage drop at the end of the line.By adding capacitors at variouslocations on the distributionfeeder, the voltage profile is sup-ported where it is needed most,allowing for substation voltage re-duction when required. On heavily

A New World Order Applying Capacitors for Peak Shaving



2 NETA WORLD

loaded distribution feeders (e.g., 18 MVA of load), theprogram has been successful in creating a peak reduc-tion of about 500 kW per circuit.

Capacitor Application for Peak Shaving(CAPS)

The CAPS strategy, developed by Cooper PowerSystems, places capacitors at multiple locations on thedistribution circuit to achieve a flattened voltage pro-file necessary for effective conservation voltage reduc-tion. Switched capacitor racks with capacitor controlsare a key part of the final solution.

CAPS allows utilities to develop a more even (flat)voltage profile that provides similar voltage to cus-tomers served at both the beginning and the end of the line. Due to the level profile, the utilities have alower kW demand during peak times, resulting in costsavings. Excess energy can then be sold back into thegrid. This also helps increase reliability, as the threatof outages due to load shedding is minimized.

Choosing the Most Effective FeedersThe CAPS program is most effective on heavily

loaded feeders since the increased magnitude of re-leased capacity makes the project more attractive.Feeders with 10 MVA or more of peak load respond best to additional capacitors. Georgia Power has beenachieving approximately 500 kW released per circuit,with average feeder loading of approximately 18MVA. Computer simulations shown in the CAPSPLUS Demonstration program show that more than700 kW of released capacity can be achieved on asample 16 MVA circuit if 70 percent of the load has aconstant impedance characteristic.

Voltage ProfileObtaining a flat voltage profile is the first step of

the CAPS program. From there, the substation volt-age can be lowered to implement CVR. Feeders thathave a significantly declining profile along the feederwill most benefit from additional capacitors. Feederswith a flat profile will not benefit from additional ca-pacitors.

Feeder Line Lengths

Additionally, short- or medium-length feeders will be most responsive to CAPS. Very long lines have in-herent voltage drop that is difficult to overcome byadding capacitors. If enough capacitors are added tosupport voltage at the end of the line, the power fac-tor will be too far leading, creating kW line losses thatwork against the effectiveness of CVR. (The programworks better if line losses are due to high demand andpoor power factor rather than the resistance of smalldiameter conductors or long lines.)

Substation Voltage SettingFeeders operating at 126 volts (on a 120-volt scale

at the substation during peak conditions will respondwell to the CAPS program. Substation voltages aroften set higher than nominal voltage (120 volts) toovercome the effects of voltage drop. Adding capacitors flattens the profile by supporting voltage at thend of the line. Since the substation voltage is highethan nominal, a flat voltage profile now makes it pos

sible to reduce the substation voltage to nominal, operhaps even lower.

Voltage DropAssuming the substation is set higher than 120 volts

if the end of line voltage is 118 volts or less duringpeak loading, capacitors will be needed to be able timplement CVR for demand reduction of any significance. If the corrected end of line voltage is 120 voltor higher, it will be possible to lower the substationvoltage by six volts, which can result in substantiademand reduction.

Power Factor A leading power factor causes line losses similar t

a lagging power factor. Therefore, the goal is to placcapacitors for voltage support while maintaining power factor near unity throughout the line. The moseffective use of capacitors for CVR is to correct thpower factor at the substation to 1.000. To accomplishthis, certain portions of the line will have a leadinpower factor, and other portions will have a laggingpower factor, but the majority of the primary feedeshould ideally be in a small band around unity, fo

example, 0.980 lagging to 0.980 leading. Circuits witha power factor of 0.90 or worse will especially benefifrom the CAPS program, since power factor will bimproved and line losses will be decreased in addition to the ability to perform CVR to release capacity

Voltage Regulation at the SubstationThe ability to lower voltage at the substation is re

quired for CVR.

SCADA Control / Radio Control / Capacitor Controls

The program requires fixed capacitors as well aswitched capacitors. More capacitors are needed timplement CVR than are needed in lower demandtimes when power factor correction may be the mainobjective. Capacitor controls may be used to switchcapacitors on when needed, or remote radio oSCADA controls may be used to switch them on whenneeded.



Spring 2002 3

Step 2

Step 3 Step 1

Voltage (120 V)

130.00

126.00

122.00

118.00

114.00

110.00

Voltage Profile

Figure 1 — Steps for Applying CAPS Program

Voltage (120 V)

130.00

126.00

122.00

118.00

114.00

110.00

Voltage Profile

Figure 2 — Base Case Voltage Profile, No Capacitors

Voltage (120 V)

130.00

126.00

122.00

118.00

114.00

110.00

Voltage Profile

Figure 3 — Voltage Profile for Intermediate Step, 6300 kvarAdded, 126 V at Substation

Voltage (120 V)

130.00

126.00

122.00

118.00

114.00

110.00

Voltage Profile

Figure 4 — Voltage Profile for New Operating Point,Substation Voltage is 123 V

Applying Capacitors for Peak Shaving(Refer to Figure 1)

1. Add capacitors to correct power factor and flattenvoltage profile.

2. Lower substation voltage to new operating point.

3. During peak times, further lower substation volt-age to reduce demand.

Customers at the end of the line are served at a volt-age similar to those customers near the substation.Compare the voltage profiles of Steps 2 and 3 (withcapacitors) to the voltage profile of Step 1 (no capaci-tors) in Figure 1.

Feeder example (all values at peak load)

Base Case (Figure 2):

• No distribution capacitors

• Voltage profile: 126 V at substation, 116.71 V at end

of line• 16 MVA nominal load (at 120 V)

• 14,718 kW + 7,989 kvar, 370 A

• 50% constant impedance load, 0.95 power factor

• 50% constant power load, 0.85 power factor

• 0.879 power factor at the substation

• 466 kW line losses

Intermediate Step (Figure 3):

Capacitors are added without changing the substa-tion voltage. This is an intermediate step between theBase Case and the New Operating Point. Figure 3 isshown to highlight the effects capacitors have on volt-age profile.

New Operating Point with Capacitors (Figure 4):

• 6300 kvar of capacitors added

• Drop the substation voltage by 3 V steady state to123 V

• Voltage profile: 123 V at substation, 118.28 at endof line

• 0.994 power factor at the substation

• 377 kW line losses

Demand increases by 9 kW, essentially no changefrom the Base Case. The system is now ready for CVRwhen needed.



4 NETA WORLD

Voltage (120 V)

130.00

126.00

122.00

118.00

114.00

110.00

Voltage Profile

Figure 5 — Voltage Profile starting at 118 V anddropping to approximately 114 V.

Final CAPS Case (CVR Employed) (Figure 5):

• 7500 kvar of capacitors total (one additional 1200 kvar band at end of line).

• Drop the substation voltage by an additional 5 V to 118 V.

• Voltage profile: 118 V at substation, 114.09 at end of line.

• 0.999 power factor

• 374 kW line losses.

• Released capacity: 528 kW

The main benefit of the CAPS program is realized here. Peak demandis reduced by 528 kW.

Case Summary

Case Substation Substation kW Losses Capacity

Voltage Power Factor Released (kW)

Base Case 126 0.879 466 n/a

New Base Case 123 0.994 377 n/a

Final CAPS Case 118 0.999 374 528

Without CAPS 124 0.877 467 252

Financial Summary

Case Capacity 36 Hours at 120 Hours at DelayedReleased (kW) $350/MWh $1200/MWh Generation at

$375/kW

Final CAPS Case 528 $6,653 $76,032 $198,000

Without CAPS 252 $3,175 $36,288 $94,500

Financial BenefitsBy implementing a CAPS

CVR program, utilities, and insome cases industry, can experience several financial benefitsPrimarily, because the programrequires less power to achieve similar output, participants canavoid buying excess power, andin some cases, may be able to in ject power back into the gridThis also helps reduce line losseand improves system efficiencsince energy is delivered tloads, not losses. Because CAPlimits the amount of peak generation capacity needed, peakingenerators may not need to badded, helping to achieve furthecost savings.

Chris McCarthy is a Senior Power Systems Engineer for Cooper Power SystemHe performs various power system analysis studies for utilities and industrial facilities, participates in software development, and is an instructor in Cooper PoweSystem’s distribution workshops. Chriholds masters degrees in electric poweengineering and business administration

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Applying Capacitors for Peak Shaving.pdf