hybrid voltage and current control approach for

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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 60, NO. 5, MAY 2013 1797

Hybrid Voltage and Current Control Approach forDG-Grid Interfacing Converters With LCL lters

Jinwei He, Student Member, IEEE , and Yun Wei Li, Senior Member, IEEE

Abstract This paper presents a hybrid voltage and currentcontrol method to improve the performance of interfacing con-verters in distributed generation (DG) units. In general, current-controlled methods have been widely adopted in grid-connectedconverters nowadays. Nevertheless, in an islanded system, thevoltage control of DG units is desired to provide direct voltagesupport to the loads. Due to the absence of closed-loop line currentcontroller, the voltage control scheme can hardly regulate the DGunits line current harmonics. Furthermore, if not addressed prop-erly, the transfer between the grid-connected operation and au-tonomous islanding operation will introduce nontrivial transientcurrents. To overcome the drawbacks of voltage- and current-

controlled DG units, this paper develops a hybrid voltage andcurrent control method (HCM). The proposed method allows thecoordinated closed-loop control of the DG unit fundamental volt-age and line harmonic currents. With the HCM, local harmonicloads of the DG unit can even be compensated without usingharmonic current extraction. In addition, the HCM guaranteessmooth transition during the grid-connected/islanding operationmode transfer. Simulated and experimental results are providedto verify the feasibility of the proposed approach.

Index Terms Active power lter (APF), current control, dis-tributed generation (DG), LC L lter, microgrid, operation modetransfer, resonant controller, selective harmonic compensation,voltage control.

I. INTRODUCTION

I N ORDER to overcome the disadvantages of fossil energy-based centralized power generation, a large number of re-newable energy sources (RESs) have been integrated into thepower distribution system in the form of distributed generations(DGs). However, the outputs of many RESs are unregulated dcpower or ac power at variable frequencies. To ensure the robustinterconnection of these RESs, the interfacing converter withLCL lter is normally placed between RESs and the main grid[1], [2].

For the control of grid-connected interfacing converters,current-controlled methods (CCMs) have been widely used

[3][9]. In this control category, the real and reactive powerreferences need to be transformed into current references. Thecurrent tracking controller is then responsible for regulating theDG current. As a standard solution for interfacing converteroperation, CCM with wide control bandwidth can effectively

Manuscript received July 29, 2011; revised December 4, 2011; acceptedFebruary 25, 2012. Date of publication March 8, 2012; date of current versionJanuary 30, 2013.

The authors are with the Department of Electrical and Computer Engi-neering, University of Alberta, Edmonton, AB T6G 2V4, Canada (e-mail:[email protected]; [email protected]).

Color versions of one or more of the gures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identier 10.1109/TIE.2012.2190374

reduce DG line current distortions. In addition, according to theemerging concept of multifunctional DG units with shunt activepower lter (APF) capabilities, DG units can also be applied toimprove the distribution system power quality by canceling theharmonic currents of local loads [8], [9].

In spite of the advantages of using CCM, the increasingpenetration of current-controlled DG units also brings someconcerns of conventional power distribution system stability[10][12]. The stability problems can be more serious when thepower distribution system switches to an off-grid system. In this

circumstance, the control mode of DG units is preferred to bechanged to voltage-controlled method (VCM) [10][15], [17].However, at the operation mode transfer instant, the conictsbetween the conventional CCM and VCM may cause nontrivialtransient currents [15], [16]. To ensure a smooth operationmode transfer, some improved methods have been developed[15], [16], where the grid-connected DG system rst reducesthe line current before switching to an islanded system. Whenthe DG system is isolated from the main grid, a voltage con-troller is immediately applied to regulate the capacitor voltageof the LCL lter. With this method, transient currents aresuppressed at the costs of a few cycles transition delay.

On the other hand, the adoption of VCM for DG units inboth grid-connected and islanding modes has no transient issueduring the grid-connected/islanding operation mode transfer,which provides opportunities to achieve the plug and playoperation of DG units in a microgrid [10], [30]. Unfortunately,as the focus of VCM is the fundamental power ow, it cannotdirectly regulate DG line current harmonics. Therefore, thevoltage-controlled DG units are usually sensitive to the dis-turbances from upstream main grid and local harmonic loads.A modied VCM-controlled DG unit was recently proposed,which realizes enhanced line current quality control through ad- justing equivalent converter series harmonic impedances [18].In this method, the performance of indirect DG harmonic line

current regulation relies on the accuracy of point of commoncoupling (PCC) voltage measurement. For a stiff grid withreduced PCC voltage distortions, it will be less effective as theextraction of PCC harmonic components is difcult.

In order to overcome the limitations of the aforementionedCCM and VCM, this paper proposes an improved DG controlmethod through the simultaneous control of the LCL lter ca-pacitor voltage and line current harmonics. Similar to the VCM,the output power of a DG unit is controlled by the regulationof fundamental lter capacitor voltage. At the same time, aclosed-loop harmonic current compensator regulates the linecurrent harmonics. Owing to the frequency selective featuresof resonant controllers [6], there is little interference between

0278-0046/$31.00 2012 IEEE


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1798 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 60, NO. 5, MAY 2013

Fig. 1. Block diagram of grid-interfacing converter systems. (a) Topology of single converter using CCM. (b) Topology of single converter using VCM.

the fundamental voltage tracking and the harmonic current reg-ulation. Further analysis on the structure of the proposed hybridcontrol method (HCM) indicates that local harmonic loads caneven be compensated without any harmonic extraction process.Finally, the DG unit using HCM can be switched between grid-connected and islanding modes at any time instant, withoutusing additional transient mitigation methods.

The rest of this paper is organized as follows. First, Section IIgives a quick review of the operation principle of DG units.The detailed discussion on the proposed HCM is providedin Section III. Further analysis and modeling of HCM-

controlled DG units in microgrids are obtained in Section IV.Section V presents the simulated and experimental results.Finally, Section VI concludes this paper.

II. DG UNIT CONTROL USING CCM A ND VCM

To better understand the principle of the proposed HCM, thetraditional CCM and VCM are briey revisited in this section.

A. Principle of CCM

Fig. 1(a) shows an example DG unit where the converter is

interfaced to PCC through an LCL lter. There are two types of loads in the system: PCC loads and the local loads placed at DGunit terminals. As illustrated, the line current I 2 is regulated bya well-known double-loop controller. In this case, the outer loopcan be a proportional plus resonant controller (PR controller) atthe stationary frame as expressed in

Gouter (s) = K P +h

Rh (1)

Rh (s) = 2K ih ch s

s2 + 2 ch s + 2h(2)

where K p is the proportional gain, K ih is the resonant gainat the fundamental and selected harmonic frequencies, h is

the angular frequency at the fundamental or selected harmonicfrequencies, and ch is the cutoff bandwidth.

Either the inductor current I 1 or the lter capacitor currentI c could be measured as inner loop feedback. For the innerloop controller, a simple proportional control as shown in (3)is normally considered, as its main function is to improve thedamping and dynamic performances of the system [4], [12],[18], [19], [23], [25], [29]

GInner (s) = K inner (3)

where K inner is the proportional gain.In order to inject power to the main grid, the synchronized

line current reference shall be calculated according to the powerreference. Additionally, for a DG unit with shunt power lineconditioner function, local load current shall also be sensed.Moreover, its harmonic component needs to be extracted andthen added to the DG line current [8], [9].

B. Principle of VCM

The topology of a voltage-controlled DG is presented inFig. 1(b). In this case, the PCC side lter inductor L2 needsto be lumped together with DG feeders. For the DG units using

VCM, the droop controllers (real power-frequency droop andreactive power-voltage magnitude droop) in the power controlstage are shown in (4) and (5) as

DG = + DP (P rated P LPF ) (4)

E DG = E + D q (Q rated QLPF )+

K C s

(Q rated QLPF ) (5)

where and DG are the nominal and DG reference angularfrequencies. E and E DG are the nominal and DG referencevoltage magnitudes. D p and D q are the droop coefcients forthe real power (P LPF ) and reactive power (QLPF ) control,respectively.

When DG units operate in the grid-connected mode, the ratedpower (P rated and Qrated ) shall be replaced by the reference


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HE A ND LI: HY BRI D VO LTAG E AN D CU RR EN T CO NT RO L AP PROAC H F OR DG-GR ID IN TE RFAC ING CO NV ERT ER S 1799

Fig. 2. Conventional double-loop current/voltage tracking in CCM and VCM.

power (P ref and P ref ). With the integral control of the reac-tive power difference in (5), zero steady-state error of the re-active power tracking can be achieved [12], [30]. Note thatthe gain (K c ) of the reactive power integral control needs tobe set to zero if the DG unit switches to islanding operation.

Therefore, DG units can share the islanded microgrid loaddemand with droop control. As the voltage control is usedfor DG units in both grid-connected and islanding operations,there is no transient current during operation mode transition(however, the microgrid and main grid should be properlysynchronized before reconnection) [17], [30].

In the voltage tracking stage, a double-loop voltage controlleris usually considered (see Figs. 1(b) and 2). Similar to theCCM, the outer loop has a PR controller. In order to reducethe capacitor voltage distortions, parallel harmonic resonantcompensators shall also be considered.

In contrast to the CCM, there is no closed-loop line currentregulation in the VCM. As mentioned earlier, the inner currentloop of VCM mainly focuses on improving system dampingrather than line current tracking. Consequently, the line currentquality is sensitive to PCC voltage disturbances and the localharmonic loads.

III. HCM

A. Proposed HCM

Although the CCM and VCM have similar controller struc-ture, to maintain proper operation of interfacing converters,their operation principles have been developed separately so far.

Due to the conicts between CCM and VCM, the outer loopsof these methods cannot be simply merged together to controlboth voltage and current.

Nevertheless, it can be noticed that the resonant controllers(2) have a high gain K ih at the selected frequency h , andthis gain decreases rapidly when the frequency is out of thebandwidth (ch ). Due to this frequency selective feature, it ispractical to control the capacitor voltage and line current atdifferent frequencies without noticeable interferences.

Furthermore, since the droop control of the DG unit wasdeveloped based on the steady-state analysis of power owbetween two voltage sources [13], the DG output power owcan be realized through fundamental capacitor voltage regula-tion using the corresponding fundamental resonant controllerR f (s). Meanwhile, the harmonic line current can be regulated

Fig. 3. Block diagram of the proposed HCM.

by the adoption of harmonic resonant controllers Rh (s) (suchas h = 3 , 5, 7, 9, and 11).

Based on the aforementioned discussion, by further incorpo-rating the cascaded structure of double-loop controller into asingle-loop parallel structure with multiple input branches, theconventional current and voltage tracking controllers for DGunit control in Fig. 2 can be replaced by the proposed hybridcontroller in Fig. 3 as

V INV = Gpower (s) (V ref V C ) + Gharmonic (s) (I ref I 2 )

+ Gdamping (s)I 1 (6)

where the rst term is a closed-loop control of fundamentalcapacitor voltage, the second term is a closed-loop control of line harmonic current, and the third term is a damping term.

The rst voltage tracking controller is shown in (7) as

Gpower (s) = GInner (s) R f (s) (7)where GInner (s) is the inner loop proportional controller inthe conventional VCM and CCM. The purpose of term (7) isto control the fundamental component of capacitor voltage, asthe reference voltage contains only the fundamental voltageobtained from (4) and (5). It has been revealed that the propor-tional gain has very limited contribution to the voltage controldynamics, and it was even removed in the controller in [24].Therefore, only fundamental frequency resonant controller isused for voltage tracking.

The second term in (6), the harmonic current controller, isdescribed as

Gharmonic (s) = GInner (s) K p +h=3 ,5 ,7 ...

Rh (s) . (8)

This harmonic current controller is used to track the har-monic current reference. As shown in (8), the selective har-monic control is adopted in this paper. Considering that linecurrents may have some noncharacteristic harmonic currents, asmall proportional gain K p is therefore used in (8) to achievebetter harmonic tracking.

Since there is no fundamental frequency resonant controllerin (8) and the gain K p is small, the line harmonic currentsand DG fundamental voltage are decoupled, and they can beregulated separately using (7) and (8). As will be illustrated


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Fig. 4. Flexible operation using HCM.

later, the term (8) is very exible, and it can be utilized to realizemany challenging power quality control tasks in an easier way.

Finally, the controller in the third term is obtained as

Gdamping (s) = GInner (s). (9)

The major function of this damping term (9) is to providegood damping to the LCL lter resonance (it can be excited bygrid disturbances, reference change, or dc link ripples).

B. Flexible Operation of DG Units Using HCM

Using the proposed HCM, a DG unit can operate with greatexibility with local load harmonic compensation mode, lineharmonic current rejection mode, etc. Furthermore, the opera-tion transition between the grid-connected mode and islandingmode can be realized seamlessly.

1) Load Harmonic Compensation: The primary aim of (8)

is to control the line harmonic current. When the referencecurrent I ref is selected to be the harmonic current of localloads, this term controls the DG unit to compensate most of the harmonics produced by local loads, leaving an improvedinjection current to PCC. Further investigation nds that (8) hasvery small gain at the fundamental frequency. Hence, to realizethe purpose of shunt APF, the measured total local current(I local ) can even be directly used as the reference current I ref in (6) without involving any harmonic extraction. This featureis particularly important for many low-power cost-effectiveDG units with limited computing capability.

2) Line Harmonic Rejection: In addition to harmonic com-

pensation ability, when I ref is set to zero, it reduces DG linecurrent harmonics. In this case, the steady-state performance issimilar to that in a DG unit with conventional CCM.

3) Seamless Operation Mode Transfer: It is also importantto note that the HCM will change back to the conventionalVCM if the input (I ref I 2 ) in (6) and Fig. 3 is replacedby (V ref V C ). Due to this feature, the challenges duringmicrogrid mode transfer can be easily solved without extraefforts. For instance, as shown in Fig. 4, when the DG unitis rst connected to the main grid, the DG unit using HCMcan inject the desired power to PCC with little line harmoniccurrents [corresponding to I ref = 0 in (6)]. Once the DG unitis disconnected from the main grid, the local harmonic loadsneed to be supplied by the DG unit with minimized voltagedistortion. To satisfy this requirement, the (I ref I 2 ) in (6)

Fig. 5. Norton equivalent circuit for DG with conventional CCM.

Fig. 6. Thevenin equivalent circuit for DG with conventional VCM.

shall be superseded with (V ref V C ) at the transition timeinstant. By using this method, a true seamless transition isachieved.

It is worth mentioning that, when the off-grid DG units needto be reconnected to the main grid, the aforementioned idea canbe used to achieve seamless reconnection in a similar way.

IV. MODELING OF DG UNITS W IT H HCM

A. Modeling of DG Unit with CCM and VCM

The conventional voltage- and current-controlled DG unitsshall be modeled using Thevenin equivalent circuit and Nortonequivalent circuit, respectively.

For DG units with the CCM, the associated Norton circuit isdescribed as

I 2 (s) = H CCM (s) I ref (s) Y CCM (s) V PCC (s). (10)

Similarly, Thevenin circuit is obtained in (11) to model the DGunit with VCM as

V C (s) = H VCM (s)

V ref (s)

Z VCM (s)

I 2 (s) (11)where H CCM (s) and H VCM (s) are closed-loop tracking gainsof CCM and VCM, respectively. Y CCM (s) and Z VCM (s) arethe closed-loop parallel and series impedances of the equivalentcircuits. The detailed modeling and analysis methods can befound in [13] and [21]. Here, the equivalent circuits for bothCCM and VCM are sketched in Figs. 5 and 6 for the readersconvenience.

B. Modeling of DG Unit With the Proposed HCM

Since the proposed HCM provides DG units with uniquehybrid voltage and current source features, the conventionalNorton or Thevenin equivalent circuits cannot be directly


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Fig. 7. Modied Norton equivalent circuit for DG with HCM. (valid atselected harmonics).

Fig. 8. Modied Thevenin equivalent circuit for DG with HCM. (valid aroundfundamental frequency).

applied to describe such a system. Instead, we propose modiedThevenin and Norton equivalent circuits to address the model-ing issues at the fundamental and harmonic frequencies.

First, the closed-loop behavior of an HCM-controlled DG

unit at the harmonic frequencies is obtained in (12) as amodied current source

I 2 (s) = H HCC (s) I ref (s) + H HVC (s) V ref (s) V PCC (s) Y DGHC (s) (12)

where the voltage reference V ref (s) responsible for fundamen-tal power control is considered as a disturbance. The coef-cients H HCC (s) and H HVC (s) are the closed-loop gains of current and voltage references, respectively. Y DGHC (s) heremeans the equivalent admittance of the parallel branch as shownin Fig. 7.

Similarly, Fig. 8 and (13) show the closed-loop characteristicof an HCM-controlled DG unit around fundamental frequencies

V C (s) = H HVV (s) V ref (s) + H HCV (s) I ref (s) Z DGHV (s) I 2 (s) (13)

where the coefcients H HVV (s) and H HCV (s) are the closed-loop gains of voltage and current references. Around the fun-damental frequency, the current reference for line harmoniccurrent regulation shall be treated as a disturbance. Z DGHV (s)is the series impedance of the modied voltage source circuit.

The detailed expressions of (12) and (13) are derived in theAppendix.

TABLE IPARAMETERS IN S IMULATION AND EXPERIMENT

C. Frequency Domain Analysis of DG Unit With HCM

In this section, the Bode diagrams of the coefcients in(12) and (13) are plotted to analyze the features of DG unitswith HCM. The control and main circuit parameters are listedin Table I.

Fig. 9 shows the current tracking gains H CCM (s) in (10) andH HCC (s) in (12). With the HCM, the closed-loop magnitude

responses at the harmonic frequencies are close to 0 dB. It isobvious that the HCM has similar line current tracking gainscompared to the conventional CCM at the selected harmonicfrequencies. It is important to note that the valid frequencyregion in Fig. 9 is the harmonic frequencies, as the DG shallbe modeled as a voltage source at the fundamental frequency.

The disturbance from the voltage reference is also examinedin Fig. 10. As shown, the disturbance has very small gains at theharmonic frequencies. Further considering that the voltage ref-erence is obtained from power droop controller (4) and (5) withvery little harmonics, the input of voltage reference disturbancehere can be ignored for line harmonic current tracking.

The effects of parallel admittances are obtained in Fig. 11.It can be seen from the gure that both Y CCM (s) in (10) andY DGHC (s) in (12) have limited gains at the selected harmonics.As a result, the line harmonic current control is not verysensitive to PCC voltage disturbances. If necessary, a com-pensator can be used to further alleviate the effects of paralleladmittances [21].

Furthermore, the voltage source behavior of the DG unitaround the fundamental frequency is studied in Figs. 1214.Note that the focus here is around the fundamental frequency.

The Bode diagrams of the closed-loop voltage tracking gainsare shown in Fig. 12. As mentioned earlier, although theproportional gain K P is removed in HCM, its closed-looptracking gain H HVV (s) has similar performance compared tothe counterpart H VCM (s) using the traditional VCM, where K p


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Fig. 9. Bode plot of H HCC (s ) and H CCM (s ) .

Fig. 10. Bode plot of H HVC (s ) .

Fig. 11. Bode plot of ( Y DGHC (s )) and ( Y CCM (s )) .

is 0.15. Consequently, the dynamics of the power control for aDG unit with HCM shall be close to that of the conventionalVCM. This conclusion will be veried by the simulation andexperimental results in the next section.

Around the fundamental frequency, the effects of current ref-erence disturbance to capacitor voltage tracking are presented

Fig. 12. Bode plot of H HVV (s ) and H VCM (s ) .

Fig. 13. Bode plot of H HCV (s ) .

Fig. 14. Bode plot of Z DGHV (s ) and H VCM (s ) .

in Fig. 13. It is clear that the disturbance has a very limitedgain around 60 Hz, and the disturbance effect can therefore beignored.

Finally, the series impedances Z DGHV (s) in (13) andZ VCM (s) in (11) are compared in Fig. 14. It shows that thesetwo impedances have similar responses around 60 Hz.


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The aforementioned analysis conrms that the DG unit withHCM shall be modeled by two decoupled equivalent circuits(see Figs. 7 and 8) at the fundamental and selected harmonicfrequencies, respectively. Note that the damping term in (6) isin parallel with both the voltage and current control branches,and therefore, it provides good damping to both the voltage

control and current control (similar to the damping providedby the inner loop of a multiloop control scheme). For thecontrol parameter design in HCM, the damping term shall berst tuned to realize a well dampened LCL lter plant [25].Then, the voltage and current control terms can be designedfor good reference tracking [28], [29]. Due to the decouplednature of the fundamental voltage control (power control term)and harmonic current control (power quality control term), theircontrol parameters can be tuned independently, as long as thegain Kp in (8) is selected to be small enough ( Kp is typicallysmall in the PR controller).

Note that another approach to simultaneously control thefundamental voltage and harmonic current is to add the addi-tional harmonic current reference into the inner current loopof a cascaded voltage control scheme [26]. To ensure goodtracking of the harmonic current reference, a high bandwidthinner loop controller (such as resonant controllers, predictivecontroller, hysteretic controller, etc.) is desired in this case.However, for the inverter with LCL lter, as the inner currentloop feedback is either the inverter side inductor current or ltercapacitor current, direct control of the line current harmonics isdifcult. Furthermore, compared to the proposed HCM scheme,the inner current controller in the cascaded control method maynot have proper damping effects, and its dynamics and stabilitywould be at disadvantage when applied to DG units with

LCL lters.

V. EVALUATION RESULTS

The performances of HCM-controlled DG units have beenveried by both simulation and experiments. The parameters of the system are provided in Table I.

At rst, a single-phase grid-connected DG system is tested inthe simulation. The detailed system conguration is illustratedin Fig. 15. To make the comparison more obvious, a dioderectier is adopted as the local load, and a resistive load isconsidered for the PCC load. Figs. 1620 show the simulated

performance of the grid-connected DG unit with different con-trol modes.When the grid-connected DG is rst controlled with the

conventional VCM, the harmonic load currents are shared bythe main grid I g and the DG unit I 2 as shown in Fig. 16,where the total harmonic distortions (THDs) of I g and I 2are 53.17% and 31.62%, respectively. In this occasion, DGcapacitor voltage is almost ripple free with only a 0.61% THD.

Fig. 17 shows the grid-connected DG unit performance whenthe HCM with local harmonic load compensation is applied(corresponding to I ref = I local in Fig. 15). As mentioned ear-lier, the harmonic extraction for conventional APFs is notnecessary in this occasion. Since the local harmonic currentis compensated by the DG unit and a linear load is placed atPCC, the main grid current I g is signicantly improved with a

Fig. 15. Diagram of a DG unit controlled by HCM.

Fig. 16. Simulated performance of DG with VCM. (a) DG line current.(b) Main grid current. (c) DG voltage.

Fig. 17. Simulated performance of DG with HCM (local harmonic compen-sation). (a) DG line current. (b) Main grid current. (c) DG voltage.


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Fig. 18. Simulated performance of DG with HCM (line harmonic rejection).(a) DG line current. (b) Main grid current. (c) DG voltage.

Fig. 19. Simulated power ow during real reference step increase.

6.85% THD. Meanwhile, the DG line current is further pollutedwith a 63.78% THD, and the capacitor voltage THD is 2.90%.

Fig. 18 demonstrates the performance of the system whenthe line harmonic current rejection (corresponding to I ref = 0in Fig. 15) is selected for the HCM. As illustrated, almost all

the harmonic currents are pushed to the main grid side, andthe THDs of I g and I 2 are 98.12% and 2.27%, respectively.At the same time, the capacitor voltage THD is 3.04%. Thisperformance is similar to the DG unit with conventional CCM,where a resonant compensator in the outer loop signicantlyreduces line harmonic currents [18].

The power tracking performances of the DG unit with bothVCM and HCM are shown in Fig. 19, where the reactive powerreference is 0 during the whole process while the real powerreference has a step increase from 30 W to 140 W at 2.5 s.The power control parameters are listed in Table I. It can beseen that the dynamics of power control using HCM are thesame as the VCM counterpart. Note that there is noticeablepower coupling in Fig. 19, and power tracking dynamics areslow. These are mainly due to the use of low-pass lters in the

Fig. 20. Simulated control mode transfer performance in grid-connectedoperation (transfer from local harmonic load compensation to line harmoniccompensation at 4.5 s). (a) DG line current. (b) Main grid current. (c) DGvoltage.

Fig. 21. Islanding microgrid with two identical DG units. (DG1 controlledwith line harmonic rejection and DG2 controlled with local harmonic loadcompensation).

power calculation/measurement and the slow integral control of reactive power error in (5). The rapid power control method in

VCM has been proposed in [27], which can also be applied toDG units using HCM.To test the response of HCM-controlled DG unit to current

reference changes, simulated results are obtained in Fig. 20,where the HCM switches from local harmonic load compen-sation mode to line harmonic current rejection mode at 4.5 s.It shows that the transition is very smooth.

The performance of HCM-controlled DG units in an island-ing microgrid is also examined by the simulation. In Fig. 21, theislanding system consists of two DG units at the same powerrating. A linear load is connected to DG1, and the nonlinearload is placed at the DG2 terminal. Both DG units are controlledusing droop control (see the controller in (4) and (5) withKc = 0 ). However, the line current control objective of DG1is to reduce its harmonics by setting I ref = 0 in (6), and


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Fig. 22. Simulated power sharing in islanding DG units. (correspondingto Fig. 21).

Fig. 23. Simulated line currents of DG units during load variation. (corre-sponding to Fig. 21).

DG2s task is to compensate its local harmonic currents usingI ref = I local .

The output power of these islanded DG units during a sudden

increase of loads is shown in Fig. 22. Similar to the islandingmicrogrid with the conventional VCM, it can be seen thatproper power sharing can always be maintained.

The associated line currents of these two DG units are shownFig. 23. It is obvious that the load current harmonics areabsorbed by DG2 (THD = 44 .32%) and the line current of DG1 contains very little harmonics (THD = 2 .25%).

The experimental results are presented to verify the proposedHCM in grid-connected operation. Figs. 2428 demonstratethe performances of a single-phase grid-connected DG unit.The circuit and control parameters are selected to be the sameas those of the grid-connected DG unit in the simulation part(see Fig. 15).

Fig. 24 shows the operation of a DG unit using the VCM.Here, the load harmonic currents are shared by the DG unit

Fig. 24. Experimental waveforms of grid-connected DG with VCM. (a) PCCvoltage. (b) Filter capacitor voltage. (c) Main grid current. (d) DG line current.

Fig. 25. Experimental waveforms of grid-connected DG with HCM (lineharmonic current rejection). (a) PCC voltage. (b) Filter capacitor voltage.(c) Main grid current. (d) DG line current.

Fig. 26. Experimental waveforms of grid-connected DG with HCM (localharmonic current compensation). (a) PCC voltage. (b) Filter capacitor voltage.(c) Main grid current. (d) DG line current.

and the main grid according to their respective harmonicimpedances [18].

The steady-state performance of the DG unit using HCMhas also been conrmed as presented in Figs. 25 and 26.


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Fig. 30. Circuit representation of an LCL lter.

supplied by the DG unit. Meanwhile, the PCC linear loads aresupplied by the main grid. To demonstrate the advantages of theproposed HCM, the disconnection time is intentionally selectedto be with the peak DG line current. It can be noticed that thereis very smooth transient during the operation mode transfer.Also, this transfer requires no additional efforts to reduce theline current to zero before the disconnection.

VI. CONCLUSION

This paper proposed a hybrid voltage and current controlmethod (HCM) for DG interfacing converters with LCL l-ters. The proposed method realizes simultaneous control of fundamental capacitor voltage and harmonic line current andtherefore can overcome the limitations of the traditional voltagecontrol method and current control method. Specically, in theproposed control scheme, the DG units fundamental powerow is regulated through the fundamental voltage tracking withdroop control, while the harmonic line current is regulatedto compensate local harmonic loads or to improve the line

current quality at the selected harmonic frequencies. Addition-ally, with the ability of both voltage and current regulationsin the developed HCM scheme, true seamless operation modetransfer between different DG compensation modes (harmoniccompensation or rejection) or different microgrid operationmodes (grid-connected operation or islanding operation) is ac-complished, which guarantees the robust and smooth operationof DG units and microgrids.

APPENDIX

A. Current Source Modeling

For the LCL lter presented in Fig. 30, the lter impedancesare expressed as z1 = L1 s + R1 , z2 = L2 s + R2 , and

zc = 1 / (C f s) + R f . Afterward, the following matrix equationcan be established according to Kirchhoffs laws as

V C (s)I 1 (s)I 2 (s)

=a11 a12a21 a22a31 a32

[V INV (s) V PCC (s)]T (A1)

where the coefcients a11 to a32 are expressed as

a11= z2 zc

z1 z2 + z1 zc + z2 zc, a12=

z1 zcz1 z2 + z1 zc + z2 zc

,

a21= z2 + zc

z1 z2 + z1 zc + z2 zc, a22=

zcz1 z2 + z1 zc + z2 zc

,

a21= zc

z1 z2 + z1 zc + z2 zc and

a32= (z1 + zc)

z1 z2 + z1 zc + z2 zc.

By substituting V INV in (A1) for (6), the closed-loop currentsource model can be derived through simple manipulations as

I 2 (s) = H HCC (s) I ref (s) + H HVC (s) V ref (s) V PCC (s) Y DGHC (s) (A2)

where the detailed expressions of the gains are shown asH HCC (s), H HVC (s), and Y DGHC (s) at the bottom of the page.

B. Voltage Source Modeling

When the DG needs to be modeled as a voltage source aroundthe fundamental frequency, the PCC side lter inductor L2 isconsidered as a part of the DG feeder. The response of an LClter can be obtained as

V C (s)I 1 (s)

= b11 b12b21 b22 [V INV (s) I 2 (s)]T . (A3)

The coefcients b11 to b22 are expressed as

b11= zcz1+ zc

, b12= z1 zcz1+ zc

, b21= 1z1+ zc

, and b22= zcz1+ zc

.

H HCC (s) = a31 Gharmonic (s)

1 + Gpower (s) a11 + Gharmonic (s) a31 Gdamping (s) a21,

H HVC (s) = a31 Gpower (s)

1 + Gpower (s) a11 + Gharmonic (s) a31 Gdamping (s) a21, and

Y DGHC (s) = a31 [Gpower (s) a12 + Gharmonic (s) a32 Gdamping (s) a22]

1 + Gpower (s) a11 + Gharmonic (s) a31 Gdamping (s) a21 a32


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H HVV (s) = b11 Gpower (s)

1 + Gpower (s) b11 Gdamping (s) b21,

H HCV (s) = b11 Gharmonic (s)

1 + Gpower (s) b11 Gdamping (s) b21, and

Z DGHV

(s) = b11 [Gpower (s) b12 + Gharmonic (s) Gdamping (s) b22]

1 + Gpower (s) b11 Gdamping (s) b21 b12

By replacing V INV in (A3) with (6), the closed-loop voltagesource model is established through a similar process

V C (s) = H HVV (s) V ref (s) + H HCV (s) I ref (s)

Z DGHV (s) I 2 (s) (A4)

where the detailed expressions of the gains are shown asH HVV (s), H HCV (s), and Z DGHV (s) at the top of the page.

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Jinwei He (S10) received the B.Eng. degree fromSoutheast University, Nanjing, China, in 2005 andthe M.Sc. degree from the Institute of ElectricalEngineering, Chinese Academy of Sciences, Beijing,China, in 2008. He is currently working toward thePh.D. degree at the University of Alberta, Edmonton,Canada.

In 2007, he was a visiting student at the National

Maglev Transportation Engineering R&D Centre,China, where he worked on the linear inductionmotor design project. From 2008 to 2009, he was

with China Electronics Technology Group Corporation. He is the author orcoauthor of more than 30 technical papers in refereed journals and conferences.His research interests include microgrid, distributed generation, and design,analysis, and control of linear electric machines.

Yun Wei Li (S04M05SM11) received the B.Sc.degree in Engineering degree in electrical engineer-ing from Tianjin University, Tianjin, China, in 2002and the Ph.D. degree from Nanyang TechnologicalUniversity, Singapore, Singapore, in 2006.

In 2005, he was a Visiting Scholar with theAalborg University, Aalborg East, Denmark, wherehe worked on the medium voltage dynamic volt-

age restorer system. From 2006 to 2007, he was aPostdoctoral Research Fellow at Ryerson University,Toronto, Canada, engaged in the high-power con-

verter and electric drives. In 2007, he also worked at Rockwell AutomationCanada and was responsible for the development of power factor compensationstrategies for induction motor drives. Since 2007, he has been an AssistantProfessor with the Department of Electrical and Computer Engineering, Uni-versity of Alberta, Edmonton, Canada. His research interests include distributedgeneration, microgrid, renewable energy, power quality, high-power converters,and electric motor drives.

Dr. Li serves as an Associate Editor for the IEEE T RANSACTIONS ONINDUSTRIAL ELECTRONICS and a Guest Editor for the IEEE T RANSACTIONSON INDUSTRIAL ELECTRONICS Special Session on Distributed Generationand Microgrids.

hybrid voltage and current control approach for

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