Volt-Var Control Using Inverter-Based

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Abstract Inverter-based distributed energy resource (DR)systems have been designed to provide constant, close to unitypower factor and thus do not provide voltage support byproducing non-active power in the form of reactive power. OakRidge National Laboratory (ORNL) has been working oncontrol schemes to provide variable reactive power which issummarized here. The Distributed Energy Communications &Controls Laboratory (DECC) at ORNL is a unique facility forstudying dynamic voltage, active power/non-active power andpower factor control from inverter-based DR. DECC interfaceswith the ORNL campus distribution system to provide actualpower system testing. Using mathematical software tools andDECC, ORNL is developing and testing local, autonomous andadaptive controls for local voltage control and active/non-activepower control for inverter-based DR. These control algorithmsare being tested using a real-time software and processorsystem interface to our computer controls interface at DECC.The control aspects of voltage regulation and active/non-activepower control using inverter-based DR are discussed andsimulation and experimental results from testing are presented.

Index Terms Distributed Energy Resources, Volt/VarControl, Reactive Power Control, Inverters, Power Electronics

I. INTRODUCTION

he concern for energy reliability and security and theenvironment has contributed to the growth anddevelopment of distributed energy resources (DR) in the

U.S. The total installed DR capacity for installations smallerthan 5MW in the U.S as of 2004 is estimated to be195GW [1]. The interconnection of DR with distributionsystems can have impacts on traditional system operationand protection [2]. The ability of DR to provide localvoltage control at the point of common coupling (PCC) is auseful ancillary service to keep the DR terminal voltagewithin accepted specified limits and thus provide voltagesupport for the distribution system/feeder. Also, voltage

D. T. Rizy, Y. Xu and F. Li are with Oak Ridge National Laboratory(ORNL), One Bethel Valley Road, Oak Ridge, TN 37831-6070.

H. Li is with Oak Ridge Associated Universities (ORAU) in the postdoctoral research associates program at ORNL.

P. Irminger is with Oak Ridge Associated Universities (ORAU) in the postBS research associates program at ORNL.

Contact: D. T. Rizy, 1-865-574-5203 (phone) or [email protected](email).This work was sponsored by the Office of Electricity Delivery & EnergyReliability, U.S. Department of Energy under Contract No. DE-AC05-00OR 22725 with UT-Battelle and conducted at ORNL and theUniversity of Tennessee, Knoxville.

support provided by DR can alleviate the reactive powershortage at the distribution and transmission levels.

DRs, such as reciprocating gen-sets, micro-turbines andfuel cells, provide a means to generate electric power closerto the end-user load more cleanly and efficiently. DRs, suchas PVs, fuel cells and most micro-turbines, that generate dcuse inverters to generate ac and interface with thedistribution system. With the right controls, these DRs cancontrol not only active power (kW) but also reactive power(kVar), to regulate local voltage near the end-user load.

DR with a power electronics inverter interface can provide both active power and non-active powersimultaneously and independently. In this way, they canregulate voltage by injecting/absorbing reactive power,correct for power factor by injecting/absorbing reactive

power while controlling their active power output, orsupport the distribution system by providing some level ofreactive power injection. They provide a faster controlcapability than capacitor banks both in terms of responseand control resolution and a more continuous output versusthe discrete output of capacitors.

Currently, inverter-based DRs are not permitted to provide local voltage regulation in accordance with the 1547

Std. [3] but a new group, P1547.8 [4], is developing arecommended practice for expanded use of 1547. An area offocus is on situations when local voltage regulation from DRshould be allowed. The impetus for this change is high DR

penetration, especially PV, expected on some distributionsystems.

II. VOLT/VAR CONTROL METHODS

It is critical to maintain an acceptable voltage range atdistribution substations (where the voltage is stepped downfrom sub-transmission level to distribution level) in order to

provide acceptable voltages to end-user loads on distributionfeeders and circuits. Voltage regulation via central generator

plants support of substations and the use of system-levelvoltage regulation equipment, such as load-tap-changing(LTCs) and capacitors, in the substations and in distributioncircuits ensures this requirement will be met. Distributionengineers dispatch capacitor banks at distribution substationsthrough manual operation or remotely controlled devices.Also, fixed and switchable (e.g., controlled by voltage)capacitor banks and line voltage regulators are used alongdistribution feeders to provide additional capability to ensurethat the end-user loads at the end of distribution feedersreceive acceptable voltage such as not less than .95 per unit.

Volt/Var Control Using Inverter-basedDistributed Energy Resources

D. Tom Rizy, Senior Member, Yan Xu, Member, IEEE, Huijuan Li, Member,Fangxing Li, Senior Member, Phil Irminger, Student Member

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978-1-4577-1002-5/11/$26.00 2011 IEEE


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With greater DR penetration, the voltage of a distributionsubstation may be regulated with DR embedded in thedemand side rather than with centralized capacitor banks atthe distribution substation or capacitor banks in thedistribution circuits. In fact, the cost of operation andmaintenance (O&M) of capacitor banks, especially thosewith controls and switchgear, is a significant burden toutilities. In addition, capacitors are least effective whenneeded the mostat low voltagessince their reactive

power capability drops off by voltage squared.A real concern is the variability of renewable DR

including wind and PV systems that dont have an energystorage component. With a low penetration of variable PV orwind, such as 10% or less, this may not be a major concern.However, when the penetration level is 20% or higher, theintermittent power source can quickly change the voltage

profile dynamically for the feeder.

A. Traditional Volt/Var ControlChanging transformer taps and switching capacitor banks

on/off are two traditional means of local voltage regulationon distribution systems. By adjusting the transformer taps,the transformer turns ratio is increased or decreased toregulate the secondary output voltage. The capability forvoltage regulation by transformers is usually within 5% ofthe rated voltage. This is the most widely used voltageregulation method in power systems, and no additionalequipment is required except transformers with tap-changingcapability (LTCs). The drawback of this method is that thetap changing is not continuous, does not respond to dynamicvoltage variation and the frequent operation increases O&M.

If voltage is regulated at a wider range, the installation ofcapacitor banks is the other traditional and widely-usedmethod. The voltage regulation effect of the capacitor banks

is not continuous and the reactive power produced by themdrops off with the square of the voltage and thus capacitors provide less voltage regulation capability at lower voltages.

These two traditional means of local voltage regulationhave some common characteristics. The capital costs arerelatively inexpensive, but the maintenance costs can besubstantial. Capacitor switching causes a transient in systemvoltage, which may have a negative impact on sensitiveloads. Neither LTCs nor capacitors can control the voltagecontinuously and they do not have dynamic capability torespond to rapid voltage variations and transients.

B. Local Voltage RegulationLocal voltage regulation refers to the ability of the end-

user to meet a voltage schedule supplied by the utility. If thecustomer were equipped with DR that could dynamicallysupply reactive power to regulate voltage, local voltagecould be regulated to maintain voltage within a requiredoperating band, such as .95 to 1.05 per unit.

DRs are not allowed to regulate voltage with reactive power control on the distribution system per the IEEE 1547Std. This restriction needs to change for high DR

penetration, especially PV-based since there are advantagesto allowing DR to regulate voltage locally and dynamically

by injecting reactive power on the distribution system. One

benefit is improving feeder capacity by reducing the reactivecurrent flow from the substation to the load andsubsequently reducing losses [5-7]. A new P1547.8 workinggroup [4] is working on a recommended practice to addressthe high DR penetration issue and to identify cases in whichlocal voltage regulation should be permitted.

C. Power Electronics-Based Volt/Var ControlLocal voltage regulation methods with real-time voltage

control capability and fast transient response are playing amore important role in the distribution level due to thedemand for high-quality power. The SVC (series varcompensator), STATCOM (static synchronous compensator),SC (synchronous condenser) and DR are some of thedevices capable of performing local dynamic voltageregulation and having the potential to provide, dynamicreactive power with the first three being system level whilethe fourth is end-user level.

There are several types of DR such as micro-turbines,industrial gas turbines, fuel cells, reciprocating enginegenerators, PVs, and wind turbines. They have great

potential for local voltage regulation by generating orabsorbing reactive power for two reasons. First, a power-electronics-based interface is required for most DR. Bymodifying the control scheme, the interface not only cancontrol active power but also reactive power from the DR tothe utility. An inverter capacity size of 10% higher can allowthe DR to provide the original active power requirementalong with reactive power for a power factor of 0.90 leadingor lagging. Second, the distributed location of DR is ideallysuited for voltage regulation.

III. DR INVERTER CONTROL

Feedback control is used in most dynamic reactiveresources, including SVC, STATCOM, and DR. Feedbackcontrol compares the differences (errors) between the actualvalues of the system variables (controlled variables) and thedesired values (references) and uses this error to determineand send control signals to the controlled system. In a robustand stable feedback control system, the controlled variablestrack the references at steady state and reach the value of thereference after a desired period of time during a transient.The time to respond to a transient is exponential, andnormally the steady-state condition is reached after five timeconstants.

A proportional-integral-derivative (PID) controller is awidely used feedback controller. There are three parts of thecontroller: (1) the proportional part that determines thereaction to the present error between the reference and actualvalue, (2) the integral part that determines the reaction to thesum of the recent errors, and (3) the derivative part thatdetermines the reaction to the rate at which the error has

been changing. A PI controller is the most commonly usedfeedback controller in which the derivative part is dropped

because it is very sensitive to measurement noise which can be aggravated by inverter operation. In the design of a PIcontroller, the proportional gain ( K p) and the integral gain( K i) are the two control constants determined by the


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characteristics of the system to be controlled and the controlobjective and strategy.

In a PI controller, inappropriate choices of the controlgain constants ( K p, K i) can result in unsatisfactory

performance of the controlled system (such as when they are both too low), system oscillation (when one or both are toohigh), or at worst system instability (when one or both areway too high). System oscillation or instability must be

avoided by selecting appropriate control gain constants. Ifall the system parameters are known, K p and K i can bedetermined by setting up the mathematical model of thesystem and the controller. Otherwise, the controller gainsmust be set by a trial/error method or by a learning process.

IV. INVERTER CONTROL SCHEMES

ORNL has developed PI control schemes for local voltageregulation and active/reactive power control of inverter-

based DR systems based on the instantaneous powertheory [8]. These schemes use a heuristic technique whichhas been verified theoretically [9-10]. Two types have beendeveloped: (1) control schemes based on fixed control gainsand (2) control schemes based on adaptive control gains.Initially, the work started with (1) but the control schemeswere not to be fast enough in responding to voltagetransients to guarantee no interference with systemequipment, such as line regulators. In (2), the control gainsare adaptive (increase/decrease) during the voltage transientto more quickly dampen it, settle to steady-state and preventvoltage instability; otherwise they are kept fixed at the initialsettings. For the active/reactive power control, the adaptivegain scheme is used.

A. Voltage Regulation with inverter-based DRThe basic idea of voltage regulation is to compare the

controlled voltage and the reference voltage and then use theinstantaneous difference (error) and the accumulateddifference as feedback signals to eventually change thecontrolled voltage to match the reference voltage within acontrol tolerance. The parameter to scale the instantaneousvoltage difference is the proportional gain, K p, and the

parameter to scale the accumulated voltage difference is theintegral gain, K i. The general PI feedback control scheme isshown in Figure 1.

Figure 1. Feedback controller for voltage regulation.

In an ideal PI control process, the response of the controlsystem to a step change is an exponential decay curve withthe largest error at the beginning of the step change and theactual variable approaching the reference. By adjusting thevalues of K p and K i, the time constant of this exponentialcurve is changed; i.e., the speed of the response is changed.

The voltage response time for the DR should be so quickthat it will not interfere with conventional utility systemvoltage control. A response time of 0.5 s (equivalent to 30cycles at 60 Hz) was selected because it should beinvisible to utility voltage control. Because 30 cycles isconsidered by utilities as an appropriate response time fordevices interconnected to the utility grid, it is used in theadaptive control as the 5 (five time constants) decay time ofthe ideal exponential curve. By adjusting the values of K p and K i so that the error of the controller tracks an idealexponential curve, the response time of the voltage control isregulated to 0.5 s. In this way the DR responds to the voltagechange and resets its reactive power output well before theconventional utility equipment begins any level of control.In other words, fast rapid control is transparent to the utilitysystem control.

B. Voltage Stability of Single DRThe voltage regulation by the inverter-based DR with

proportional-integral (PI) control is shown in Figure 2a-dwith different combinations of control gains ( K p, K i). Theresponse of the local voltage regulation can be affected by

the selection of the control gains. In these figures, thevoltage regulation is using the reference value of 268V inthe simulation. The PCC voltage was 263V beforeregulation. From 0 to 0.3 s, there is no regulation; from 0.3sto 1.5s, compensation is performed by the DR to regulate thevoltage. In the figure, the straight green line is the referencevoltage, and the blue plot is the voltage at the point ofcommon coupling (PCC).

Figure 2a shows a stable but slow voltage response whenthe gains are too low. The voltage isnt able to reach thereference value even after 1.3s. Figure 2b shows the voltageregulation with an overshoot and oscillations for some time(at least 0.9s) before settling due to the gains being high.

Figure 2c shows an unstable voltage regulation due to thegains being too high with oscillations swinging from 266Vto 270V after 1.2 s. Figure 2d shows a desirable dynamicresponse with well-designed gains but when the controlgains are outside this range, the system becomes unstable, asshown in Figure 2c.

C. Current Limiting ControlLimiting the current output of the inverter is important in

the case of a large transient, such as a large change involtage [10]. If the current output were not limited, theinverter would trip out on over-current protection when thevoltage change was severe. This is desirable only in case ofa local fault; otherwise, it is preferred that the inverter

produce up to its capability to continue supporting localvoltage to the best of its ability. Without this controlcapability, the inverter would trip out and any support that itcould provide would be lost. For current control, the inverterswitches from voltage regulation to current regulation whenthe inverter current starts to exceed its rated or preset limit.Thus, current control prevents the inverter from tripping outwhen a large transient occurs unless the transient is a fault.

Current regulation works much like voltage regulation:the average rms current for all three phases is calculated andcompared with a current limit. If the current is below the


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limit, the inverter switches back to voltage regulation mode.However, if the current comes near or reaches the limit, it isregulated to the reference current. The control switches backto voltage regulation mode only when the current drops

below the limit again.

0.3 0.6 0.9 1.2 1.5260261262263264265266267

268269270

time(s)

v o l t a g e ( v )

(a) Voltage regulation with low gains ( K p=0.01, K i=0.1) .

0.3 0.6 0.9 1.2 1.5260261262263264265266267

268269270

time(s)

v o l t a g e ( v )

(b) Voltage regulation with high gains ( K p=0.2, K i=3.0).

0.3 0.6 0.9 1.2 1.5260261262263264265266267268269270

time(s)

v o l t a g e ( v )

(c) Unstable voltage regulation due to gains ( K p=1.0, K i=7.0)

being too high.

0.3 0.6 0.9 1.2 1.5260261262263264265266267268269270

time(s)

v o l t a g e

( v )

(d) Voltage regulation with correct gains ( K p=0.03, K i=0.8 ).

Figure 2. Voltage regulation with different control gains forsingle DR.

D. Balanced Voltage Regulation with adaptive gains andcurrent control

Figure 3 shows the configuration for an inverter-basedDR interfaced with the utilitys distribution system. Theupper portion shows a power system, which is simplified asan infinite bus with a utility voltage (v s), system impedance( L s, R s) and an RL load. The lower portion is the DR with itsinverter, controller and coupling inductor Lc. The output

voltage and current of the inverter are designated as vc andic, respectively.The control approach [9] uses a PI control as shown in

Figure 4 to dynamically regulate the average voltage of thethree-phase voltages to the reference level. PI controlcalculates the error between the average and the referencerms voltage to determine the reference output voltage of theinverter. Pulse width modulation (PWM) signals aregenerated based on the reference output voltage and fed tothe switch gate-drives of the inverter. The average rmsvoltage, updated at every sample, is the average of the three

phase voltages. When the magnitude of the voltage produced by the inverter is higher than the system voltage, the inverteris injecting reactive power, and the inverter current isleading the system voltage. In this case, the inverter israising the local voltage. When the magnitude of the voltage

produced by the inverter is lower than the system voltage,the inverter is absorbing reactive power, and the invertercurrent is lagging the system voltage. In this case, theinverter is lowering the system voltage.

DR

Figure 3. Inverter-based DR interfaced with utility distributionsystem.

Figure 4. PI controller diagram.

Normally the gains of the PI controller, in this case the proportional gain ( K p) and the integral gain ( K i), stayconstant for voltage regulation regardless of whether thesystem is in steady state (very little change in voltage) ortransient (a large voltage change such as due to a large


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motor start or load step change). Normally, the best gainvalues are determined and left fixed for the control.However, it is helpful for the gain to change dynamically(adaptively) during a transient, depending upon the severityof the voltage error (difference between actual and referencevoltage). The techniques for both adaptive and non-adaptivecontrol are described in [9-10].

Figure 5 shows the (1) no voltage regulation, (2) balancedvoltage regulation with fixed gains and (3) balanced voltageregulation with adaptive gains in response to a local voltagetransient of 2V.

(a) No voltage regulation by inverter in response to transient.

(b) Voltage regulation with fixed control gains.

(c) Voltage regulation with adaptive control gains.

Figure 5. Balanced voltage regulation of inverter-based DR.

The experimental results shown are from testing atORNLs Distributed Energy Communications & Controls

(DECC) Laboratory 1. In the figure, the straight red line is thereference voltage, and the blue plot is the voltage at the pointof common coupling (PCC).

The voltage reference for the balanced voltage regulationusing non-adaptive gains is 266.5V. The voltage referencefor the balanced voltage regulation using adaptive gains is268V. They are different since they were tested on differentdays under different distribution system operatingconditions. The voltage regulation without adaptive gainsshows that it takes just under 4s to reach steady-state againfollowing the transient. However, in the case of the voltageregulation with adaptive controls, it is much faster and takesonly 0.5s to reach steady-state again following the transient.

Adaptive gain control employs a step response for avoltage change that closely fits the exponential idealresponse and reduces the voltage error to zero in five timeconstants. The time constant depends on the responsecapability of the DR. The PI gains change only during thetransient; they are thus unchanged before and after thetransient event. Limits are set on the gains to prevent voltageovershoot or undershoot during the transition and to avoidvoltage instability (voltage oscillations) as well.

E. Unbalanced Voltage RegulationThe approach for unbalanced voltage regulation uses the

rms voltage calculation for each of the three phase-to-neutralvoltages and compares each to the reference (one setting forall three) to determine the voltage errors for each phase.Each phase of the inverter is controlled independently tocorrect the voltage error.

To compensate for the unbalance in each phase, the three- phase voltages are controlled separately, instead of theaverage value of the three-phase voltages, which is thecontrol scheme in section IID. However, the requirements ofthe inverter current ratings, the inverter dc side voltage

rating, and the inverter dc side capacitor rating are higherthan the balanced voltage regulation case.In power systems, most voltage unbalance conditions are

due to magnitude inequalities while the phase-angles areequal (120 or 2 /3) or nearly equal. If the voltageunbalance is in this category and the inequalities in themagnitudes are not very large (less than 1%), the DR with aninverter interface can compensate for the voltage unbalance

by providing reactive power. By controlling the three-phasemagnitudes of the inverter voltage, vc, individually, the rmsvalues of the three phase PCC voltages, vt , are controlled at agiven reference level.

Figure 6 shows results for the unbalanced voltagecompensation. Figure 6a shows the waveforms of the three-

phase rms voltages (V rms) and the reference voltage (267.7V). The high-frequency switching (12.5 kHz) which wasused to control the inverter creates some EMI noise on thesignal but this can be eliminated with better shielding for theinstrumentation. As shown in Figure 6, from t = 0 to t = 7.1s,the inverter is operated in the unbalanced voltagecompensation mode and after t = 7.1 s, it is turned off. Whilethe inverter is on and operating, the three-phase voltages are

1The DECC Lab provides real-world testing/operation for inverter-basedDR since it is interfaced to the ORNL campus distribution system which isone of the 159 TVA distributors.


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regulated at the reference level and nearly balanced, andwhile the inverter is off, the voltages are unbalanced andlower than the reference.

Figure 6b shows the three-phase rms inverter currents(A rms) produced by the inverter to balance and regulate thevoltages. The maximum current is 70A (phase c) while theminimum current is only 20A (phase b).

(a) PCC line-to-neutral voltages (V rms).

(b) Inverter phase currents (Arms).

Figure 6. Unbalanced voltage compensation control.

F. Active and Reactive Power ControlA decoupled control algorithm of active power and

reactive power has been developed [10]. Combining adecoupled control with a current limiter ensures that theinverter is not overloaded. Using this DR algorithm, theinverters active current and reactive current are controlleddirectly, simultaneously, and independently.

The active power has higher priority over the non-active power, and maximum non-active power is generated withinthe inverters capability. This allows the energy from a DRto be fully transferred to the grid and non-active power to be

provided to the grid. This control algorithm allows theinverters capabilities to be taken full advantage of at alltimes, both in terms of functionality as well as within theinverters capability such as its kVA rating. 2

The active power, P (t ), and reactive power, Q(t), can becontrolled indirectly and independently by controlling theactive current I ca(t ) and reactive current I cn(t ), which are therms values of the instantaneous active and reactive current.

2 The inverter may be de-rated to account for additional thermal heatingdue to high-speed switching to provide faster dynamic performance.

The decoupled feedback control diagram is shown inFigure 7. In the reactive power control loop, the amplitudeof the instantaneous inverter output voltage, vc(t ), iscontrolled by the PI controller PI 1, where I cn

* is the invertercurrent reference, I cn is the actual value, and K p1 and K i1 arethe proportional and integral gains of PI 1. Using the PCCvoltage as its reference, the amplitude of the inverter outputvoltage is modified based on the amount of the reactive

power. The result of this control loop is v*c1(t ), which is in phase with the PCC voltage vt (t ).

Figure 7. Active and reactive power control diagram.

The inverter active power control is controlled byadjusting the phase angle of the inverter output voltage. The

phase angle of vc(t ) is controlled by the PI controller PI 2,where I ca

* is the active current reference, and I ca is the actualvalue.

The active current reference I ca* and the non-active

current reference I cn* can be calculated according to the

preset active power reference and reactive power reference[10]. By adding the current limiter to the controller, theactive current reference and the reactive current referencesettings are modified so that the inverters current will notexceed the current limit. The inverter can be controlled to

provide variable or fixed active power along with variable or

fixed reactive power. In other words, both active andreactive power can be controlled at all times. Examples ofactive and reactive power control under two differentscenarios are shown next.

1) Active Power Control with Reactive Power Held Fixed: Active power control with a set level of reactive power output is shown in Figure 8. The active powerreference (P ref ) is increased from 10 to 50 kW, and the non-active power reference (Q ref ) is set at 10 kVar. Figure 8a-cshow the active power (P), the reactive power (Q) and theinverter rms current (i c). The current is the average of thethree phase-to-neutral values of the inverter. In Figure 8a-b,the red lines are the active power reference and reactive

power reference, respectively. The blue plots in the figuresare the actual P and Q measured during the operation of theinverter.

Before the active power reference is changed, the invertercurrent is below the current limit (60A). When the invertercurrent exceeds the current limit, the current limiter istriggered. The active and reactive current references arecalculated based on the current limit so that the total currentis limited to 60 A.

2) Reactive Power Control with Active Power Held Fixed: Reactive power control that is variable with a setlevel of active power output is shown in Figure 9. The active


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power reference (P ref ) is set to 20kW while the reactive power reference (Q ref ) increases from 20 to 50 kVar. Figure9a-c shows the active power (P), the reactive power (Q) andthe inverter rms current (i c). As earlier, the current is theaverage of the three phase-to-neutral values of the inverter.

(a) Active power and reference.

(b) Reactive power and reference.

(c) Inverter current.

Figure 8. Variable active power control with fixed non-activepower.

When the P and Q references are 20 kW and 20 kVar, theactual values are tracking the references very well. However,when the reactive power reference is increased to 50 kVar,the reactive power increases until the inverter currentreaches the current limit. Thus, the actual reactive power islower than the reference since the limit is exceeded. As

indicated above, the current limit is triggered after thecurrent reaches above 60 A.

(a) Active power and reference.

(b) Reactive power and reference.

(c) Inverter current.

Figure 9. Variable reactive power control with fixed activepower.

G. Smart Inverter ControlUltimately, the goal is to have an inverter-based DR that

can be commanded via communications depending uponsystem conditions to change control modes. During normalconditions, the inverter may be controlled to provide somelevel of reactive power along with operating at some active

power level. If the reactive power is kept at a fixed level, itwouldnt regulate voltage but would offset the reactive

power needed from the substation. The reactive power onthe other hand could be varied to regulate voltage to


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maintain a reference setting within some tolerance. Thereference could be communicated to the DR, such as

provided by the utility, to achieve a desired voltage profile based either on calculations from a model or measured data points or both.

Also, the control of multiple inverter-based DR systems,which provides active control of voltage or active/reactive

power, needs to be understood. We are completing a secondinverter-based DR test system at DECC which will be usedto develop and conduct testing of multiple systems.

V. CONCLUSIONS

A summary of using a local feedback control scheme forinverter-based distributed energy resource (DR) todynamically regulate local voltage while limiting current is

presented. The scheme that we use is near-real time andcalculates the rms of the local voltage in every sample anduses it as input for a PI controller for the inverter PWM tovary reactive power output. The feedback controller operates

by comparing actual voltage based on the calculated rmswith a reference setting, which can be based on a utility-specified voltage schedule, to determine the voltage error.The error and PI controller gains determine the new outputfor the inverter.

The controller also can provide current control. While theinverter is in voltage control mode, the current control modeactivates when the control calls for current beyond theinverters capability or to which it has been limited. Theinverter will switch from current mode to voltage mode onlyif the actual inverter current is below the current limit. Thecurrent control mode ensures that the inverter does notoperate beyond its rated (thermal or electrical) capability andthus ensures continuous support of system voltage forvarious voltage transients unless a nearby fault necessitatesthat the inverter trip out or in accordance with protectioncoordination.

The control scheme for balanced voltages can maintainthe voltage within 0.1 V of the reference voltage.However, unbalanced voltage regulation requires moreinverter current output capability and higher dc link voltagealong with some additional computations; instead of one rmscalculation as in the balanced mode, there are now three. Butthe extra calculations do not impact the unbalanced controlimplementation.

Adaptive PI control continuously calculates what newgain is needed during a voltage transient to follow theexponential time response curve of the inverter based onvoltage error and time. However, it changes the gain only

when a significant difference between the actual andreference voltage values exists, such as greater or equal to1 V. Otherwise, the gain is kept fixed since, the transient is

perceived to be mild enough to be handled by the existing PIgain settings. Adaptive control gains ensure a time responseof 0.5 s which prevents DR interaction with conventionalutility system equipment for regulating system voltage.

VI. REFERENCES[1] F. Li, J. Kueck, T. Rizy, and T. King, 2006, A Preliminary Analysis

of the Economics of using Distributed Energy as a Source of Reactive

Power Supply, Oak Ridge National Laboratory (ORNL), Oak Ridge,TN, ORNL/TM-2006/014 , Apr. 2006.

[2] J. Driesen and R. Belmans, Distributed Generation: Challenges andPossible Solutions, IEEE PES General Meeting , pp. 8 , 2006.

[3] Information on the 1547 series of interconnection standards can befound at http://grouper.ieee.org/groups/scc21/dr_shared/ . The 1547standard is available from the IEEE Standards Association athttp://standards.ieee.org/findstds/standard/1547-2003.html .

[4] See http://grouper.ieee.org/groups/scc21/1547.8/1547.8_index.html .[5] D. T. Rizy and R. H. Staunton, Evaluation of Distribution Analysis

Software for DR Applications, ORNL/TM-2001/215 , September2002, http://certs.lbl.gov/pdf/eval-of-der-02.pdf .

[6] John D. Kueck, Brendan Kirby, D. Tom Rizy, Fangxing Li, and Ndeye Fall, "Reactive Power from Distributed Energy," The Electricity Journal , vol. 19, no. 10, pp. 27-38, December 2006.

[7] D. T. Rizy, F. Li; H. Li; S. Adhikari and J. D Kueck, "ProperlyUnderstanding the Impacts of Distributed Resources on DistributionSystems," 2010 IEEE PES General Meeting, pp.1-5, July 2010.

[8] Yan Xu, L. M. Tolbert, J. N. Chiasson, F. Z. Peng, J. B. Campbell, "AGeneralized Instantaneous Nonactive Power Theory for STATCOM,"

IET Electric Power Applications , vol. 1, no. 6, pp. 853-861, Nov.2007.

[9] H. Li, F. Li, Y. Xu, D. T. Rizy and J. D. Kueck, Adaptive VoltageControl with Distributed Energy Resources: Algorithm, TheoreticalAnalysis, Simulation, and Field Test Verification, IEEE Transactionson Power Systems , vol.25, no.3, pp.1638-1647, Aug. 2010.

[10] Y. Xu, H. Li, D. T. Rizy, F. Li, and J. D. Kueck, "Instantaneous activeand nonactive power control of distributed energy resources with acurrent limiter," 2010 IEEE Energy Conversion Congress andExposition (ECCE), pp.3855-3861, Sept. 12-16, 2010.

VII. B IOGRAPHIES

D. Tom Rizy (SM87) is a senior research power systems engineer at OakRidge National Laborat ory (ORNL) in the Power and Ene rgy SystemsGroup. He is a cofounder of the Distributed Energy Communications andControl Laboratory (DECC) at ORNL for testing dynamic voltageregulation controls using distributed energy resources. He has over 33years experience in power systems R&D and received his MSEE and BSEEfrom Virginia Tech and the University of Virginia, respectively.

Yan Xu (S02M06) received the B.S. degree from Shanghai JiaotongUniversity, Shanghai, China, in 1995, the M.S. degree from North ChinaElectric Power University, Beijing, China, in 2002, and the Ph.D. degree in

electrical engineering at The University of Tennessee, Knoxville, in 2006.She is currently a research staff member at Oak Ridge National Laboratory(ORNL), Oak Ridge, TN.

Huijuan Li (S07, M10) is working as POSTDOC associate for Oak Ridge National Laboratory under the ORAU program. She received her Ph.D. inelectrical engineering from The University of Tennessee in 2010 and herB.S.E.E. and M.S.E.E. in electrical engineering from North China ElectricalPower University, China in 1999 and 2002 respectively. She previouslyworked as a research engineer at Shanghai Sieyuan Electrical Company inChina on the field of ungrounded neutral distribution systems.

Fangxing (Fran) Li (M01, SM05) received the Ph.D. degree fromVirginia Tech in 2001. He has been an Assistant Professor at TheUniversity of Tennessee (UT), Knoxville, TN and an adjunct researcher atORNL since August 2005. Prior to joining UT, he worked at ABB, Raleigh,

NC, as a senior and then a principal engineer for four and a half years. Hiscurrent interests include energy market, reactive power, and distributedenergy resources. Dr. Li is a registered Professional Engineer in NorthCarolina.

Phil Irminger is an MS student in Electrical Engineering at The Universityof Tennessee and is working as a POSTBS associate for Oak Ridge

National Laboratory under the ORAU program. He received his B.S(Electrical Engineering) from The University of Tennessee, Knoxville in2009.

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