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Revision of IEEE Standard 1547™New Reactive Power and Voltage Regulation Capability RequirementsBY REIGH WALLING, WALLING ENERGY SYSTEMS CONSULTING, LLC.DECEMBER 2016

What has changed?IEEE Standard 1547, which defines interconnection requirements for distributed energyresources (DER), is presently undergoing a major revision. One of the areas in the standardthat is being changed dramatically involves the requirements regarding DER reactive powerand voltage regulation. The present (2003) version of this standard imposes no reactivepower capability requirements and prohibits active voltage regulation by DER. The newstandard, now in the final draft stage, includes major reversals on these requirements.

What is the impact on cooperatives?Reactive power capability and control functions that deploy reactive power are a potentiallypowerful tool that can be used to mitigate the voltage impacts of DER interconnections, if these functions are applied correctly. If applied incorrectly, these functions may alsointerfere with proper feeder voltage regulation and can result in voltage instability. Also, the use of DER reactive power capability to mitigate voltage rise caused by DER power exportcan reduce a cooperative’s power factor and increase losses. Developers proposing tointerconnect DER to cooperatives may request to use this capability in lieu of more costlyoptions to reduce voltage impacts, such as feeder reconductoring. Cooperatives need tounderstand the pluses and minuses of using these new capabilities.

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This article is the second in a four-part series regarding the IEEE standard 1547 and its impact onthe electric grid. The background and purpose of this standard was reviewed in the first article,Revision of IEEE Standard 1547™ — The Background for Change. This article focuses on relatedissue of voltage regulation; and subsequent articles will discuss disturbance performance andpower quality. A primary purpose of this series is to ensure cooperatives are well informed of theimportance of this standard and the upcoming related balloting session, and of the opportunity to be involved in the process to ensure their perspective is reflected in upcoming changes to thestandard. Details on how to participate in the balloting process are defined in the first article.

s business and technology strategiestransmission and distribution strategies work group

nreca members only

Continued

https://www.cooperative.com/interest-areas/engineering/Documents/tsieeestandard1547pt1backgroundnov2016.pdf


Revision of IEEE Standard 1547™ — New Reactive Power and Voltage Regulation Capability Requirements | 2

INTRODUCTIONFor the past thirteen years, IEEE Standard1547™-2003 has been the model distributedenergy resource (DER, i.e., distributed genera-tion or storage) interconnection standard usedwidely across the U.S. and Canada. This stan-dard is now undergoing major revision to bringit up to date with the changes in DER technol-ogy and DER penetration levels seen in manyplaces across the country. The standard revi-sion process is now in the final draft stage, withthe proposed standard scheduled for industryballot in early 2017.

Some of the major changes proposed for thestandard are with regard to DER reactive capa-bility and the control functions necessary to deploy this reactive power. This TechSurveil-lance article is the second in a series of fourthat describe the changes that are proposed forthis standard, provide the rationale for thesechanges, and describe how they will affect theplanning, design, and operation of rural electriccooperative distribution systems into the future.This article focuses on the reactive power andvoltage regulation related changes in the newIEEE 1547 draft. Although it is now in the finaldraft stage of the development process, cooperative engineers are encouraged to be-come involved in the review and balloting of

this proposed standard revision to ensurethat it sufficiently addresses their system cir-cumstances. (Details on how to participate areavailable in the first article.)

INCREASING DER PENETRATION IMPACTS VOLTAGESThe incremental current flow from DER, no mat-ter how limited the penetration, has some effecton the voltage levels along the feeder. When thepenetration1 of DER becomes large, unaccept-able voltage impacts can result. This penetrationcan be either from one, or a few, large DER facili-ties, or the aggregate result of many small DER,such as behind-the-meter rooftop solar photovoltaic (PV) units. DER can create two differentkinds of voltage impact. One is changes in thevoltage profile along the feeder; the other is variability of voltage due to DER that have rapidvariations in their output, such as solar PV anddistribution-connected wind generation.

Voltage Profile Impacts of DER Voltage magnitude drop due to load on a sectionof distribution feeder is approximately equal tothe real current (kW/kV) times section resist-ance plus lagging current (kVAR/kV) times sec-tion reactance. Shown in equation form, this is:

When the penetrationof DER becomes large,unacceptable voltageimpacts can result.

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What do cooperatives need to know or do about it?Cooperatives need to understand the implications of the proposed reactive power and voltageregulation changes to IEEE Standard 1547, including the opportunities these changes provide, aswell as their potential pitfalls. Rural electric cooperatives often have long distribution feeders withrelatively small conductors and long single-phase taps; situations that are less frequentlyencountered by investor-owned utilities and municipalities. As the standard’s draft developmentis now concluding, co-op engineers are encouraged to join the ballot pool when it opens toensure that the standard adequately addresses the needs of the cooperative community.

1 DER “penetration” is a measure of how much DER is connected relative to the characteristics of a system. There are anumber of ways in which DER penetration can be quantified. Each of these metrics has particular relevance to differentDER impacts. Examples are aggregate DER capacity relative to peak circuit load, minimum circuit load, and minimumdaytime circuit load (the latter particularly applicable to PV).

V • R + • XPV

QV



DER output is, for purposes of voltage evalua-tion, equivalent to a negative load. So, the flowof power from a DER into the distribution sys-tem results in a voltage rise due to its interac-tion with circuit resistance. Most DER today areoperated at unity power factor, so the discus-sion of DER reactive power will be addressedlater in this article.

When DER power output exceeds the load demand downstream of the point of intercon-nection to the feeder, power flow will reverse at

that point on the feeder. As a result, voltage willrise with increasing distance along the feederfrom the substation. This is opposite of the nor-mal tendency for voltage to steadily decline withincreasing distance, except where a mid-linevoltage regulator or capacitor bank is used toboost the voltage. If the DER output is largeenough, voltage can be pushed above the upperlimits of the voltage range allowed by ANSI C84.1.Figure 1 shows the voltage profile for an exam-ple case where a large DER is interconnected atthe end of the feeder, compared with voltageprofile on the same feeder without the DER.

DER can also result in low voltage conditionson a distribution feeder, primarily by interactingwith the line-drop compensation (LDC) func-tions of feeder voltage regulator or substationtransformer tapchanger controls. LDC adjuststhe voltage regulation setpoint, based on thecurrent measured at its location, in order to accommodate normal feeder voltage drop. This function is based on the assumption thatthe flow measured at the regulator or substa-tion is representative of the loading situationdownstream along the feeder. A large DER unitlocated downstream, but near, to the regulatingdevice results in a decreased or reversed flowat the device. As a result, the amount of voltagesetpoint boost provided by the LDC is reduced,and voltage may not be sufficient at that loca-tion in order to keep voltage at the far endabove the minimum acceptable level. This situ-ation is illustrated in Figure 2.

Simultaneous high and low voltage situationscan occur when large DER penetration is pres-ent on one feeder of a substation where thebus voltage is regulated using LDC. The flowthrough the substation transformer is reducedby the DER, causing less LDC boost to thetapchanger control. The voltage at the bus maybe too low to provide adequate voltage at theend of other feeders without DER, but at thesame time, the bus voltage may be too high

If the DER output islarge enough, voltagecan be pushed abovethe upper limits of thevoltage range allowed

by ANSI C84.1.

130

128

126

124

122

120

118

116 0 2 4 6 8

Distance (miles)

Voltage (120

V base)

With DERWithout DERANSI C84.1 Upper LimitANSI C84.1 Lower Limit

FIGURE 1: Example of a large DER at the end of a feeder pushing voltageabove limits.

128

126

124

122

120

118

116

114 0 2 4 6 8

Distance (miles)

Voltage (120

V base)

With DERWithout DERANSI C84.1 Upper LimitANSI C84.1 Lower Limit

DER Location

FIGURE 2: Example of DER interacting with substation transformertapchanger control to depress feeder voltage below limits.


such that the feeder with DER may have excessvoltage at the DER connection point.

The present IEEE 1547-2003 version prohibitsDER from causing voltage at any other customer’sservice point outside the limits of ANSI C84.1Range A. Accommodating high DER penetrationcan sometimes require modification of feedervoltage management practices (e.g., tap andcapacitor bank switching control setpoints),physical relocation of capacitor banks or regu-lators, or even feeder reconductoring in orderto avoid excessive voltage. A cooperative willavoid making any changes that can reduce thequality of service to other customers, so theopportunities for simple low-cost solutions,such as control setpoint changes, are limited.

Voltage VariationA DER can have objectionable impact even if itdoes not drive voltage outside of the ANSIC84.1 limits. DER that have inherent variabilityin their output, specifically solar PV and windgeneration, can cause very frequent voltagevariations. Figure 3 shows an example of PV facility power output, comparing a clear “bluesky” day with a partly-cloudy day where cloud

shadows frequently pass over the solar arrays.The resulting voltage variations are often termed“flicker” but in most realistic situations, thevoltage variations produced by solar and windgeneration are not sufficiently rapid or large tomeet the definitions of objectionable flicker asdefined in IEEE Standard 1453™. IEE1453 isbased solely on human perception of incandes-cent lamp flicker and does not address othervoltage variation impacts. Instead, the voltagevariations from PV and wind become objection-able by causing too-frequent operation of coop-erative voltage regulating devices, such asfeeder voltage regulators, substation trans-former on-load tapchangers, and capacitorswitches. The resulting excessive operationscan require increased maintenance costs forthese cooperative devices and may result inpremature device failure.

Voltage variability due to solar and wind DERcan also cause temporary excursions outside ofacceptable voltage ranges, because the voltageramps can be much faster than the responsetime of mechanical feeder voltage regulationequipment. Decreasing the device operationcounts by using wider regulation deadbands orlonger delay times tend to aggravate the tem-porary voltage excursion problem. Voltage vari-ability problems often require feeder reconduc-toring or may cause a DER interconnectionrequest to be denied.

REACTIVE POWER AS MITIGATIONThe costs of feeder modification to accommo-date a DER project, particularly large modifica-tions such as reconductoring, are typically theresponsibility of the DER project proponent. Insome states, these requirements are includedin interconnection regulations. Therefore, par-ties seeking to interconnect, as well as cooper-atives wishing to accommodate DER while pro-tecting their other members from adverseimpacts, desire alternative ways to mitigateDER voltage impacts.

Voltage variability due to solar and wind DER can also cause

temporary excursionsoutside of acceptable

voltage ranges.

5

4

3

2

1

0 6:00 AM 10:00 AM 2:00 PM 6:00 PM 10:00 PM

Hour of Day

Power (M

W)

Clear DayCloudy Day

FIGURE 3: Typical PV facility power output during clear and partly-cloudydays. (Source: R. Walling, DistribuTech 2012)


Reactive power absorption by DER interactswith feeder reactance to create a voltage dropthat opposes the voltage rise caused by the interaction of active (i.e., real) current withfeeder resistance. Both rotating generators andinverters are inherently capable of both reac-tive power production and absorption that aresmoothly variable and can be changed fre-quently without wear and tear, such as that experienced by mechanical voltage regulationequipment. Reactive power can be changed almost instantaneously by inverters and withina few seconds by rotating generators. Reactivecurrent, therefore, can be used to mitigate boththe steady-state voltage impacts and voltagevariability impacts of DER.

The use of DER to mitigate DER-caused voltagerise is not without its own adverse impacts. Anyreactive power consumed by DER must be replaced elsewhere in the cooperative system.The source of this replacement reactive powerwill normally be at or near the substation endof the feeder in order for it to be effective inraising the substation power factor without increasing feeder voltage near the DER. Thismay necessitate installing capacitor banks at or near the substation in order to maintain thepower factor of the substation.

This may be counterintuitive for those who workprimarily with correcting power factor for typicalloads, where capacitors are added closer to theloads on the feeder to help reduce losses andincrease voltages. With DERs, adding capaci-tors near the DER would still improve the sub-station power factor, but voltages near the DERcould be increased beyond ANSI C84.1 limits.

Means must also be provided to control the DERreactive power in order to achieve the voltageimpact mitigation objective. The various controltechniques vary in their effectiveness, applicationcomplexity, and potential for interference withother cooperative voltage regulation schemes.

INCREASING SYSTEM IMPACTS DRIVESTANDARD CHANGESDER penetration growth over the last decadehas been explosive in many parts of the US.Most of this growth has been in solar PV, in theform of both utility-scale power generating facilities in the MW range and behind-the-meter retail installations. Many utilities todayhave distribution feeders where the connectedDER capacity exceeds the load demand. Conse-quently, critical voltage impacts created by DERhave become a more prevalent issue.

The levels of DER penetration seen in manyplaces today are far greater than were com-monly foreseen when the original IEEE 1547was adopted in 2003. Although it was knownthen that DER reactive power could reduce volt-age impacts, it was also realized that coordina-tion of this capability with utility voltage regula-tion complicates the interconnection process.In order to minimize the amount of utility anddeveloper interaction, IEEE 1547-2003 prohib-ited DER from “actively regulating” the voltageat the point of interconnection. Although thishas often been misinterpreted as a requirementthat DER only operate at unity power factor (noreactive power flow), the intent of the standardwas to prohibit controls that vary reactivepower in response to measured voltage. As aconsequence of this prohibition, DER has beendenied interconnection in some cases whereconstructive use of DER reactive power wouldeliminate objectionable voltage impacts and, inother cases, the DER developers have had topay for feeder reconductoring or constructionof a dedicated feeder in order to interconnect.

The use of DER reactive power can sometimesbe a satisfactory alternative to expensive distri-bution system upgrades. Today’s DER situationis far different than in 2003, and the draft revi-sion of IEEE 1547 reverses the stance on DERreactive power capability and voltage regula-tion. The new standard will instead require DER

Reactive current,therefore, can be

used to mitigate boththe steady-state

voltage impacts andvoltage variabilityimpacts of DER.

The levels of DERpenetration seen in

many places today arefar greater than werecommonly foreseen

when the original IEEE1547 was adopted.


to have reactive power production and absorp-tion capability and the DER must possess a num-ber of prescribed control functions in order toregulate this reactive power. And, because newDER will be required to have these capabilities,the cooperative can demand its utilization whennecessary to address feeder voltage issuescaused by DER. It must be emphasized, how-ever, that this reactive power and voltageregulation functionality is to be utilized onlywith the express consent of the cooperative.

Point of Applicability

Now that the reactive power and control func-tionalities will be required, the point where therequirements will apply has become a morecontroversial subject. Providing a certain amountof reactive power at the DER terminals is notequal to providing the same amount at thePoint of Common Connection (PCC) with the cooperative system, due to reactive powerlosses (e.g., from transformer reactance) andgains (e.g., due to solar farm cable charging capacitance) between these locations. Like-wise, regulating the DER terminal voltage to acertain value does not result in the PCC voltagebeing regulated to that value.

DER vendors prefer to have the requirementsapply at the DER terminals because they wantto market self-contained systems that are certi-fied to be compliant without regard to the details of the customer’s balance of system(i.e., transformers and cables between the DERand the PCC). Utilities, on the other hand preferto have capabilities defined at the boundariesof their system, and do not wish to consider thedetails of the customer’s system. After muchdebate, the IEEE P1547 working group2 has

reached a compromise where “retail” DER inter-connections will have the requirements applyat the DER terminals and “wholesale” DER willhave requirements apply at the PCC.3 “Whole-sale” DER are facilities where the aggregate DERrating exceeds 500 kW, and the annual averageload demand of the facility is less than 10 percentof the rating. The intent is to cover pure genera-tion facilities, with some allowance for auxiliaryloads, security lighting, etc. All other DER facili-ties, including DER rated greater than 500 kW,but within a facility having significant load,would be considered “retail” and the require-ments are applicable at the DER terminals.

VOLTAGE LIMITSThe existing IEEE 1547-2003 requirement thata DER not cause any other customer’s servicevoltage to go outside of ANSI C84.1 Range A is retained in the draft P1547. However, addi-tional limitations are also applied. The DERcustomer’s own PCC voltage must also not gooutside of Range A, unless the customer isserved by a dedicated transformer. This is toensure that future customers added to thesame secondary system have acceptable service voltage. Another new requirement isthat the cooperative’s primary voltage shouldalso not be driven outside of Range A.

REQUIREMENTS FOR REACTIVE CAPABILITYThe draft P1547 requires DER to have reactiveproduction and absorption capability in pro-portion to the DER kVA rating, over the range of power output from 20 percent power up tofull power rating. The DER can produce activepower (kW) in excess of the power rating, up to the kVA rating, as long as it remains capableof meeting the reactive power requirements if

After much debate,the IEEE P1547 working

group has reached acompromise where“retail” DER inter -

connections will have the requirements apply at the DER terminals and

“wholesale” DER will have requirements apply at the PCC.

It must be emphasizedthat this reactive powerand voltage regulationfunctionality is to be

utilized only with the express consent of the cooperative.

2 The “P” in P1547 designates a standard under development. After the standard draft is developed and successfully balloted, the P is dropped from the standard number. The P1547 now under development, when adopted, will replace the existing IEEE 1547-2003.

3 The terms “retail” and “wholesale” are not used, per se, in the standard. However, these terms are used in this article for clarity to reflect the intent of the standard.


demanded, presumably by reducing the activepower in order to maintain the DER within itsapparent power (kVA) rating.

IEEE 1547 has been, and will continue to be atechnology-neutral document. However, the in-herent ability of certain types of DER to meetreactive power requirements differ, and settingrequirements that exclude some DER technolo-gies would not be acceptable. In many cases,the potentially-excluded technologies performbeneficial roles that cannot feasibly be per-formed by other technologies. Specifically, syn-chronous generators have limitations to theirreactive absorption capability (under-excitedoperation). Synchronous generators, however,are the preferred technology for recovering en-ergy from biogas, backup generation, and otherfunctions beneficial to society. Because IEEE isa technical group, it is outside of their scope tomake policy decisions based on total societalbenefit. Therefore, the standard has defined

“performance categories” that apply to the reac-tive power and regulation functions describedin this article, as well as disturbance perform-ance requirements that will be discussed in thenext article in this series. For reactive powerand control requirements, two categories havebeen defined. Category A has requirementsthat are feasible for all known DER technologiesto meet. Category B has enhanced requirementsthat are beneficial to the power system, butmay not be achievable by all technologies. Thedraft standards punts on the decision regardingwhich performance category will be required onthe basis of application and DER technology,deferring this decision to the “Authority Gov-erning Interconnection Requirements” (AGIR).The AGIR could be a regulatory agency, orcould be the cooperative itself.

The proposed reactive power capability require-ments for Category B are for reactive powerfrom 44 percent of the DER nameplate appar-

ent power (kVA) rating in the kVAR absorbing direction to 44 percent ofnameplate kVA in the kVAR producingdirection. This is equivalent to 0.9power factor at rated load. However,this is not strictly a power factor require-ment because the DER must be able toinject or absorb this same amount ofreactive power for power levels as lowas 20 percent. The reactive power capa-bility requirements for Category B DERare depicted in Figure 4. The semicircu-lar area for power greater than rated kWis the optional area where a DER mayoperate if the reactive power that is demanded at that moment is within the region. If greater reactive power isrequired, the DER may need to reduceits active power in order to supply thereactive power. For Category A, the kVAR-absorption requirement is reduced to25 percent of the nameplate kVA, pri-marily in order to accommodate syn-chronous generators.

IEEE 1547 has been,and will continue to

be a technology-neutral document.

44% of kVA(capacitive)

Active Power

Mandatory Reactive Capability

OptionalContingentOperation

+Q

P

–Q

Reactive Pow

er

No Reactive Capability Required

20% of kW(18% of kVA)

100% of kW(90% of kVA)

111% of kW(100% of kVA)

44% of kVA(inductive)

FIGURE 4: Reactive power capability requirements for Category B DER.


The 20 percent minimum power level for reac-tive capability has its roots in interconnectionrequirements used around the world for renew-able resources. The original intent of these otherrequirements was to accommodate wind plantsat low wind speeds where only a few turbineshave wind speed above the turbine startupvalue and, thus, the overall plant has reducedcapability to meet reactive power requirementsunder marginal wind conditions. The rationaleis that all wind turbines can be assumed to beon line when 20 percent power level is reached.The extension of this lower power limit to DERin IEEE P1547 has raised some concerns of volt-age steps when a DER’s output dithers aboveand below this 20 percent power threshold andthe reactive capability could cut in and out.

REGULATION MODESFor reactive power to be applied for DER volt-age impact mitigation, the reactive power mag-nitude needs to be controlled automatically.This is particularly true for DER having inher-ently variable output, such as PV. A reactivepower magnitude that is good for one powerlevel may adversely affect voltage at a differentDER output.

There are a wide variety of distribution systemdesigns and situations, so there is no one con-trol scheme that fits all circumstances. P1547,therefore, specifies a number of different con-trol modes that can be applied to deploy reac-tive power in the best way to meet system objectives. These control modes are:

• Constant power factor

• Constant reactive power

• Reactive power as a function of active power(watt-VAR mode)

• Reactive power as a function of voltage (volt-VAR mode)

• Volt-VAR mode with voltage reference tracking

Both Category A and B DER are required tohave each of the above control modes avail-able, with the exception that the watt-VARmode is optional for Category A.

The choice of DER control mode and its param-eters are to be at the discretion of the coop-erative. The proposed standard defines rangesof control parameters for which the DER musthave full adjustability, and a default value is pro-vided for each parameter that is to be used wherethe cooperative has not specified otherwise.

Constant Power Factor

In the constant power factor mode, reactivepower is directly proportional to the active (kW) power. The power factor is settable between 0.9 in the kVAR producing direction to 0.9 in the kVAR absorption direction.4 Thiscontrol mode is both simple and, at the sametime, quite effective if the correct power factorsetting is chosen. It can be shown mathemati-cally that a constant kVAR-absorbing power fac-tor equal to the sine of the arctangent of the reactance to resistance ratio (sin(Tan-1(X/R)) ofthe cooperative system results in cancellationof the voltage changes caused by DER powervariations. The voltage variations, however, areonly nulled at the point where the power factoris held constant (DER terminals for retail appli-cations, and the PCC for wholesale generatingfacilities). Voltage may still vary elsewhere inthe system, particularly if there is a large differ-ence in the X/R ratio of the system impedanceat any point along the distribution system toward the substation.

This is a situation where the requirement forwholesale generating facilities to achieve theirperformance at the PCC is particularly beneficial.Such facilities typically have their PCC at theprimary voltage level. At points along primaryfeeder ahead of the PCC, the X/R ratio is most of-ten reasonably constant except where there is a

A reactive powermagnitude that is

good for one powerlevel may adverselyaffect voltage at a

different DER output.

The choice of DERcontrol mode and itsparameters are to beat the discretion ofthe cooperative.

4 The definitions for leading and lagging power factor for generation are opposite of the definitions for loads. In order toavoid confusion between load and generation power factor conventions, leading and lagging are not used here.


very large change in conductor size or a transi-tion from overhead to underground. This allowsthe constant power factor mode to largely miti-gate most of the DER facility’s voltage impact.

For retail DER connected at the secondary level,there is frequently a large change in the upstreamX/R ratio between the secondary and primarysides of the service transformer. For example, alarge three-phase transformer may have an X/Rratio of ten, but the primary feeder’s X/R ratio ismore typically around two or three. The constantpower factor can be set such that the second-ary voltages variations are nulled, but primaryvariations are not. This is less of an issue whensmall DER are interconnected because they, onan individual basis, do not usually have signifi-cant primary voltage impact. Where there ishigh penetration of small DER, utilities mayhave voltage variability issues at both second-ary and primary levels.

The constant power factor control mode is an“open-loop” control function, meaning the response of the cooperative system does notaffect what the function does. As a result, thismode is unlikely to adversely interact with thecooperative system’s voltage regulation con-trols. One exception is when the cooperative’sfeeder capacitor banks are controlled on thebasis of measured reactive current flow on afeeder. The reactive power absorbed by the DERin order to cancel resistive voltage rise may alsocause the cooperative capacitor banks to switchon with consequent voltage increase. This defeats the purpose of the power factor mode.

Constant Reactive Power Mode

In the constant reactive power mode, the DERinjects or absorbs a constant amount of reactivepower independent of the active (kW) power.An exception is where the active power dropsbelow 20 percent of rating, at which point theDER is no longer required to have reactivepower capability (it may hold the reactivepower, or it may not, depending on the DER

design). Effectively, this makes the DER act likea capacitor or shunt reactor from the reactivepower standpoint. There may be special cir-cumstances where this mode is useful, but it isnot likely to have wide application except forDER that have constant power output. One suchspecial circumstance can be where a coopera-tive has an integrated volt-VAR control (IVVC)and reactive power levels are dispatched on acontinuous basis by the IVVC to individual DER.This is an advanced “smart grid” concept thatmight be applied in the future, and having thissimple function available in DER is desirable,even if it has very limited application today.

Reactive Power as a Function of Active Power

The “watt-VAR” function varies reactive poweraccording to a defined function of the active(kW) power. This function differs from the con-stant power factor mode because the relation-ship can be nonlinear. The relationship is defined by a two-slope line. Figure 5 illustratessome of the Q vs P characteristics that can bedefined within the range of parameter adjust-ments. Some example application cases forthis function are to:

• Minimize cooperative system reactive bur-den when the DER output is moderate to low, but increase the reactive power absorp-tion at high er levels of active power in orderto mitigate voltage rise. (Example shown inFigure 5a.)

• Compensate for the reactive power losses ofthe service transformer reactance, so that aDER following this curve on the secondaryside produces a roughly constant power fac-tor on the primary side, thereby minimizingprimary voltage impact. (Example shown inFigure 5b.)

• Provide the cooperative system with lagging(i.e., capacitive) reactive power except whenthe DER output is high and it must go tounity power factor to avoid creating an over-voltage. (Example shown in Figure 5c.)

Where there is highpenetration of smallDER, utilities may

have voltagevariability issues atboth secondary and

primary levels.


“Volt-VAR” or Voltage Regulation Mode

Perhaps the most powerful of the required reactive power regulation modes specified inthe draft P1547 is the “volt-VAR” mode in whichthe reactive power output is a function of themeasured voltage (at the DER terminals in thecase of retail applications, and at the PCC forwholesale generation applications, as definedpreviously in this article). Figure 6 illustratesthe reactive power versus voltage characteristicused for this control mode. The parameters

defining this characteristic are required to havea substantial range of adjustability. Therefore,the function can be customized to a particularapplication situation. As with all of the reactivecontrol modes discussed in this article, the parameter settings, as well as whether thismode is even used, are to be at the discretionof the cooperative.

Because reactive power affects voltage, andvoltage affects the reactive power injected orabsorbed by the DER, the volt-VAR mode is a“closed-loop” control. Effectively, this mode isessentially the same as the voltage regulationfunction that has been used with synchronousgenerators for the last century. The referencevalue for the voltage regulation, i.e., the voltagethat the control attempts to achieve, is the volt-age at which the specified characteristic yieldszero reactive power. The parameters of the func-tion allow definition of a deadband; a range ofvoltages for which there is no change in the reactive power. Usually, the reactive power inthe deadband region is zero and the reference,or target voltage, can be considered to be thecenter of the deadband range. On each side ofthe deadband is a slope; the amount of reac-tive power change for a given change in meas-ured voltage. This, from a control engineering

+Q

Reactive Pow

er

P

–Q(a)

+Q

Reactive Pow

er

P

–Q(b)

+Q

Reactive Pow

er

P

–Q(c)

FIGURE 5: Example watt-VAR mode characteristics.

+Q

Reactive Pow

er

DeadBand

VoltageReference

V

–Q

FIGURE 6: Reactive power versus voltage characteristic of the voltageregulation mode.

previous view


standpoint, is the “gain” of the regulation func-tion. If this gain is too high (i.e., the slope is toosteep), voltage regulation from the DER maybecome unstable. The gain where instability occurs is proportional to the stiffness of thesystem. Regulator instability can cause oscilla-tory variations in the voltage and should defi-nitely be avoided.

This volt-VAR mode is powerful, because it canautomatically regulate distribution voltage towards a desired value, but also requires con-siderable engineering attention because of theinstability issue mentioned above and the poten-tial for this function to interfere with other feedervoltage regulation schemes if not carefully coor-dinated. The draft standard has default param-eter settings recommended, which should generally be safe from stability issues in mostcases. But, even these settings can interferewith voltage regulation. For example, the defaultsetting for the reference voltage is the nominalvalue. It is typical practice for a cooperative torun the head of a feeder at higher than nominalvoltage in order that the far end is within theacceptable range. A DER connected near thehead of the feeder using the volt-VAR modewith default parameters will fight with the intended voltage profile by absorbing reactivepower in an attempt to pull the feeder headvoltage towards nominal (1.0 p.u. or 120 V). A different interaction can be caused by a largeDER in volt-VAR mode located near a feedervoltage regulator, which causes the regulatorcontrol to “hunt,” continuously raising and lowering the tap setting. Interactions can alsooccur with voltage-controlled capacitor banks.

Engineering studies are needed to properly apply the DER volt-VAR mode. There is an inherent tradeoff between the aggressivenessof the function and the risks of unintended con-sequences. It may be possible, however, for a cooperative to develop standardized applica-

tion rules that allow the mode to be success-fully applied without detailed study of the spe-cific DER interconnection. For example, an ap-plication rule might be to set the voltagereference according to the DER’s location onthe feeder. For feeders that may be part of aloop scheme, the location of the DER relative to the head and tail of the feeder will changedepending on which end the feeder is ener-gized at any given time. This can make thechoice of acceptable volt-VAR mode parame-ters more difficult.

Voltage Regulation Reference Tracking

The draft P1547 also specifies a variation of thevolt-VAR mode in which the reference value, orregulation objective, tracks the long-term aver-age of the actual voltage. This mode adapts tothe prevailing voltage, so that the DER does notfight with the feeder voltage profile but insteadpreserves the dynamic range of the DER’s reac-tive power to mitigate shorter-term voltagevariations. This can be a very effective way toaddress the propensity of large PV installationsto cause excessive regulator, tapchanger, andcapacitor switch operations. However, this modedoes not allow the DER to correct long-termvoltage profile issues caused by DER penetration.In many cases, it may be more efficient to ad-dress long-term voltage issues with regulatorsand capacitor banks, and reserve the DER’sability to make fast and frequent reactive power changes for the short-term volt-age impacts.

Voltage-Power Mode

There is one control function specified in P1547that is related to voltage regulation but doesnot involve reactive power. The “volt-watt” modelimits active (kW) power in response to highvoltage conditions. This mode is not intended for routine voltage regulation as the frequentlimitation of power can have a profound eco-nomic impact on the DER owner. Instead, this

There is an inherenttradeoff between theaggressiveness of thefunction and the risks

of unintendedconsequences.


function is intended to address excessive volt-age levels that might be caused by DER powerexport. The standard recommends a defaultvalue of 105 percent of nominal voltage (126 V)for the beginning of power limitation; this param-eter is to be adjustable from 103 to 110 percent.The value of voltage where power export is lim-ited to zero is adjustable up to 110 percent.Thus, this function can be considered to be a“partial trip” function that is perhaps a betteralternative for the distribution system than anabrupt trip of the entire DER output when amoderate overvoltage threshold is reached.(Rapid tripping for more severe overvoltage remains in the standard and will be discussedin the next article of this series.)

CONCLUSIONSInterconnection of large DER facilities, or highlevels small DER penetration, can cause signifi-cant cooperative voltage issues, both in termsof out-of-limit voltages and voltage variability.The revision of IEEE 1547 provides a solutiontoolbox by mandating DER to have reactivepower capability, along with a variety of controlfunctions by which this reactive power can bedeployed to mitigate voltage issues. AlthoughDER reactive power is very effective, use of it to reduce voltage rise caused by DER power export tends to increase distribution systemlosses and can reduce the system’s net power

factor at the substation. Some degree of dis -tribution engineering attention is needed to implement DER reactive controls depending on regulation mode utilized and the degree of mitigation aggressiveness desired.

DER reactive capability is to be used only asapproved by the cooperative, and the controlmode parameters are to be specified by the cooperative. These changes to the standardcan be helpful to cooperatives in many cases.Options will have to be evaluated and settingsdeveloped for different situations. The toolsprovided will be available if needed or desired,but the revised standard does not change theeffective status quo if the cooperative does notagree to use the reactive control tools.

With the draft P1547 now being finalized, theopportunity for participation in draft develop-ment has come to a close. However, there is anopportunity for more co-op engineers to partic-ipate in the standard balloting process. TheIEEE-SA standards balloting process, and theprocedure to join the ballot pool was describedin the previous TechSurveillance article in thisseries. Review of the new IEEE 1547 by co-opengineers and participation in balloting willhelp to ensure that the needs of the rural cooperative segment of the power industry are appropriately addressed. n



About the Author

Reigh Walling is a utility and renewable energy industry consultant, focusing his practice on thetechnical issues related to DER interconnections and renewable energy integration, as well as avariety of transmission-related areas. He has long been heavily involved in standards related tointerconnection of DER and transmission-scale renewable energy plants, including participation inthe inner writing group of the original IEEE 1547, several of the IEEE 1547.x companion standards,NERC PRC-024, and the NERC Integration of Variable Generation Task Force, and as well as a co-facilitator in the current IEEE 1547 revision working group. Prior to establishing Walling EnergySystems Consulting in 2012, he was a key member of GE’s Energy Consulting group for 32 years.While at GE, he was the program manager for the Distribution Systems Testing, Application, andResearch utility consortium, of which NRECA is a long-standing member.

R. Walling, R. Saint, R. Dugan, J. Burke, L. Kojovic, “Summary of Distributed Resources Impacton Power Delivery Systems”, IEEE Transactions on Power Delivery, Vol. 23, No. 3, July, 2008.

R. Walling, Z. Gao, Eliminating Voltage Variation Due to Distribution-Connected RenewableGeneration, 2011 DistribuTECH Conference, San Diego, Feb. 2011.

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The Transmission and Distribution Strategies Work Group, part of NRECA’s Business and Technology Strategies team, is focused on identifying opportunities and challengesassociated with efficient, reliable electricity delivery by cooperatives to consumers.TechSurveillance research relevant to this work group looks at the various aspects oftransmission and distribution grid infrastructure technology and standards. For moreinformation about technology and business resources available to members through theTransmission and Distribution Strategies Work Group, please visit www.cooperative.com,and for the current work by the Business and Technology Strategies department of NRECA,please see our Portfolio.

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Legal Notice

This work contains findings that are general in nature. Readers are reminded to perform due diligence in applying thesefindings to their specific needs, as it is not possible for NRECA to have sufficient understanding of any specific situationto ensure applicability of the findings in all cases. The information in this work is not a recommendation, model, orstandard for all electric cooperatives. Electric cooperatives are: (1) independent entities; (2) governed by independentboards of directors; and (3) affected by different member, financial, legal, political, policy, operational, and otherconsiderations. For these reasons, electric cooperatives make independent decisions and investments based upon theirindividual needs, desires, and constraints. Neither the authors nor NRECA assume liability for how readers may use,interpret, or apply the information, analysis, templates, and guidance herein or with respect to the use of, or damagesresulting from the use of, any information, apparatus, method, or process contained herein. In addition, the authors andNRECA make no warranty or representation that the use of these contents does not infringe on privately held rights. Thiswork product constitutes the intellectual property of NRECA and its suppliers, and as such, it must be used in accordancewith the NRECA copyright policy. Copyright © 2016 by the National Rural Electric Cooperative Association.

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