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Distributed Operation of Interlinked AC Microgrids with Dynamic Active and Reactive Power Tuning Inam Ullah Nutkani, Member, IEEE, Poh Chiang Loh, Senior Member, IEEE, and Frede Blaabjerg, Fellow, IEEE
Abstract—Microgrids are small grids formed by clustering modern generating sources, storage systems, and loads together. Being independent, the formed microgrids can, in principle, operate at their own preferred voltages and frequencies. Tying them to the mains grid or another microgrid would therefore require some interlinking power converters, whose control should preferably be autonomous without depending on fast communication links. Contributing to this theme of research, a distributed power management scheme has been proposed in this paper for interlinking two or more independent microgrids operating at different voltages and frequencies. The proposed scheme allows sources in the microgrids to concentrate more on active power harnessing, while the interlinking converters focus more on meeting the load reactive demand. If necessary, backup active power from an underloaded microgrid can also be transferred to an overloaded microgrid, allowing it to supply loads in excess of its rated capacity. The performance of the proposed scheme has already been tested in experiment. Index Terms—AC–DC–AC power conversion, distributed generation, microgrids, power converters.
I. I NTRODUCTION
I
NCREASING global demand for energy has led to the fast adoption of distributed generation and microgrids powered by unconventional sources like photovoltaic, wind, tidal, geothermal, and high-speed diesel generators [1]–[5]. The interconnection of these distributed sources has greatly been made possible by the rapid advancement in power converters, whose appropriate control has led to improvement in power quality, efficiency, and design flexibility [6]–[9]. The third factor, when related to microgrids, means that they can be designed with their own preferred nominal voltages and frequencies that would better suit their source and load characteristics. This has,
Manuscript received October 14, 2012; accepted December 22, 2012. Date of publication May 7, 2013; date of current version September 16, 2013. Paper 2012-IPCC-569, presented at the 2012 IEEE Energy Conversion Congress and Exposition, Raleigh, NC, USA, September 15–20, and approved for publication in the IEEE T RANSACTIONS ON I NDUSTRY A PPLICATIONS by the Industrial Power Converter Committee of the IEEE Industry Applications Society. I. U. Nutkani is with the Experimental Power Grid Centre, Agency for Science, Technology and Research (A*STAR), Singapore 138632, and also with the School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798 (e-mail:
[email protected]). P. C. Loh is with the School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798 (e-mail: epcloh@ ntu.edu.sg). F. Blaabjerg is with the Department of Energy Technology, Aalborg University, 9220 Aalborg East, Denmark (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIA.2013.2262092
in fact, been done on board electric ships and aircraft if their distribution grids are viewed as small microgrids designed with their own preferred voltages and frequencies [10], [11]. The formed microgrids can, of course, operate independently, but for greater security and reliability, they are usually intertied by power converters. The intertied microgrids can, in turn, operate as an island or connected to the utility grid through a static switch or an ac–dc–ac power converter, as shown in Fig. 1. The intertying of grids is no doubt not a new concept that has commonly been done with the utility grids for reserve sharing, provision of emergency supply, and energy trading [12]. It has also been demonstrated recently in [13] that the forming of different intertied networks operating at their optimal voltages can help to reduce transmission and distribution losses, hence triggering an interest in intertied systems. Although the intertying concept is not new, its application to microgrids can be more challenging because of a number of related operating issues. The first is their usual demand for autonomous control without fast communication link because of their widely dispersed source placements. The second is their more stringent supply–demand balance enforced within each comparably sized microgrid, which surely is more complex than the case studied in [14]. In that study, only a single microgrid is connected to the utility grid, treated as an infinite bus or ideal source, whose loading does not affect the overall control. Another case can be found in [15], where two or more single-phase microgrids are tied together. The study there, however, focuses more on improving performances of the individual power electronic gateways like minimizing their dc-link power pulsations and improving their dynamic responses. These objectives are no doubt important given that they have been mentioned in nearly all power electronic applications. They are, however, not directly related to the intertying of microgrids. Rather, power flow management among the microgrids might be a more relevant topic, whose implementation would usually involve the interlinking power converters and energy sources operating autonomously. A possible scheme is now proposed for realizing the intertying, whose first requirement is to adaptively distribute the system generation responsibilities among the sources and interlinking converters, depending on their present loading conditions. The adjustment causes energy harnessed from the sources to be maximized, the operating time and losses of the converters to be minimized, and reserve sharing to be realized with no installed backup capacities. Details of the scheme and its accompanied experimental testing can be found in later sections of this paper.
0093-9994 © 2013 IEEE
NUTKANI et al.: DISTRIBUTED OPERATION OF INTERLINKED AC MICROGRIDS
Fig. 1.
Intertied ac microgrids operating at different voltages and/or frequencies.
Fig. 2.
Active and reactive droop lines for microgrids and interlinking converters.
II. S OURCE C ONTROL W ITHIN I NDIVIDUAL M ICROGRIDS To avoid fast communication links, sources in a microgrid can be controlled by the conventional droop method, whose operating principles have been clarified by many researchers, including those in [16]. Variations for compensating feeder impedances and other mismatches can also be found in [17]– [21] and will hence not be repeated. Instead, the focus here should be more on the formulation of modifications to the basic droop method that can further improve power generation when multiple microgrids are intertied. To do that, the droop lines shown for microgrid “x” (= “a” or “b”) in Fig. 2 are reviewed first, whose expressions for source unit “y” are written as follows: ∗ fxy = fx,max + γxy Pxy ∗ Vxy = Vx,max + ϑxy Qxy
(1)
where superscript “∗” and subscript “max” have been added to represent the reference and maximum values, respectively.
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∗ ∗ and Vxy are therefore the microgrid reference Variables fxy frequency and voltage determined from their maximum values and measured active and reactive powers. Also notated in (1) are γxy and ϑxy , representing the active and reactive droop coefficients (or gradients of the droop lines), respectively. The droop coefficients are small negative values, which, for cases of multiple sources (including energy storage systems), should be set according to
γx1 Sx1,max = γx2 Sx2,max = γx3 Sx3,max = . . . ϑx1 Sx1,max = ϑx2 Sx2,max = ϑx3 Sx3,max = . . .
(2)
where Sxy,max represents the source kVA rating. With (2) enforced, sources within the microgrid will share the active and reactive power demands proportionally based on their ratings. This works fine in a single microgrid, where the sources and source converters are the only generators of active and reactive powers. However, with multiple microgrids intertied, interlinking converters placed between any two of them can serve as additional generators of active and reactive powers,
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Fig. 3. Realization of microgrid droop-controlled source.
whose operating constraints may not be the same as that of the sources. Modifications to the droop tuning principles to incorporate the different power generator characteristics may therefore result in improved performances. In particular, for the sources within each microgrid, their active power generation should be prioritized at high load to draw the maximum amount of energy from the distributed sources. To remain within their converter kVA limits, their reactive power generation must correspondingly be reduced. The reduced reactive power is instead supplied by the interlinking converters to avoid unnecessary load shedding. The interlinking converters can supply active power too but would usually be less efficient, as compared to reactive power. To illustrate, Fig. 2 is referred to, where it is assumed that microgrid “a” demands some active and reactive powers from the interlinking converters. Providing reactive support would require operating interlinking converter “a” only, but to transfer active power without varying the dclink voltage, both interlinking converters “a” and “b” must be operating. It can hence be more efficient if the intertied system is coordinated such that the sources concentrate more on active generation, while the interlinking converters concentrate more on reactive generation to keep their operating losses low. With the aforementioned approach defined, active droop coefficients of sources within microgrid “x” should be tuned according to (3) and not (2) γx1 Px1,max = γx2 Px2,max = γx3 Px3,max = . . . Pxy,max = Sxy,max 2 − (Qxy,max |P xy,max )2
eral, be chosen such that critical loads can ride through periods of interlinking converter failure or scheduled maintenance. The shedding of noncritical loads is, however, necessary if loading is high during these periods of interlinking shutdown. Complementing (3), the reactive droop coefficients should be tuned according to (4), where an expression for computing Qxy,max at different Pxy is also given. Since Qxy,max is varying, the droop coefficients or gradients are also varying, as seen from the reactive droop lines drawn on the left and right of Fig. 2 for microgrids “a” and “b” ϑx1 Qx1,max |P x1 = ϑx2 Qx2,max |P x2 = ϑx3 Qx3,max |P x3 = . . . Qxy,max |P xy = Sxy,max 2 − P xy 2 ≤ Sxy,max . (4) The above droop equations can be implemented based on the control block arrangement shown in Fig. 3. Also shown in the figure is the usual double-loop control, whose design and realization have been discussed in the literature [7], [8] and hence are not duplicated here. The only difference noted with the figure is the regulation of grid currents iA , iB , and iC instead of those flowing through the filter inductors Lf . The latter are usually used but would require three additional current sensors. For simplicity, the grid currents can be measured and subtracted by an estimate of the filter capacitor currents iCf,ABC in (5) to get the estimated filter inductor currents iCf,ABC + sCf νABC /(1 + s/ωcut )
(3)
where Qxy,max is a function decreasing with the increase in active generation Pxy . Notation Qxy,max |P xy,max is therefore the maximum reactive power that source unit “y” can produce when it is at its maximum active generation of Pxy,max . The value for Qxy,max |P xy,max is user defined and should, in gen-
(5)
where vABC represents the source terminal voltages. Also included in (5) is a low-pass filter with cutoff frequency of ωcut for reducing noise amplification. III. C ONTROL OF I NTERLINKING C ONVERTERS As drawn in Fig. 1 (and other figures in this paper), the simplest interlinking converters would consist of two back-to-back
NUTKANI et al.: DISTRIBUTED OPERATION OF INTERLINKED AC MICROGRIDS
six-switch converters, whose functionalities can qualitatively be described as follows. 1) To complement reactive power generation in the microgrid, particularly under high loading conditions, during which the source reactive power has been reduced according to (4). 2) To transfer active power from an underloaded microgrid to an overloaded microgrid. Reserve sharing is thus effected without introducing extra backup generators. 3) To properly regulate power flow so that their operating time and, hence, losses are minimized. This can be done by, for example, generating no extra active and reactive powers when both microgrids are underloaded and hence does not require additional support. Interlinking powers should also be zero when both microgrids are heavily overloaded and hence do not have extra generating capacities for sharing. Realizing the aforementioned functionalities would strictly require the interlinking converters to “know” the consolidated source generating conditions of each microgrid. Here, consolidated, rather than individual, source conditions are mentioned because, at the terminals of the interlinking converters, individual details within the two microgrids cannot possibly be seen. Because of that, subscript “y,” representing individual source unit, will be omitted from subsequently derived droop expressions. Returning to the consolidated source or microgrid generating conditions, they can be detected by measuring the microgrid frequencies and voltages, which are linearly related to active and reactive powers according to (1). Different thresholds can then be set for the frequencies and voltages to distinguish when the microgrids are underloaded (UL), overloaded (OL), or heavily overloaded (HOL) in terms of active and reactive powers. Based on the classifications, the interlinking converters can promptly make the appropriate power flow decisions using only variables measured locally. The interlinking droop scheme is hence autonomous, whose operating details are presented shortly. A. Interlinking Reactive Droop Characteristics In Section II, it is mentioned that reactive power generation in microgrid “x” will be reduced at high active loading conditions. The amount reduced must instead be supplied by interlinking converter “x,” whose kVA rating Sx,IC,max must be higher than that after incorporating a comfortable margin to permit some active power transfer at high reactive loading, if desired. On the other hand, when the microgrid reactive loading is low, reactive support from the interlinking converter is not needed since sources in the microgrid can promptly meet the load reactive requirements. The interlinking converter can hence be turned off to minimize its operating losses.
Q∗x,IC
⎧ ⎨ 0, = σx (Vx − Vx,OL ), ⎩Q x,IC,max
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With these considerations incorporated, two interlinking reactive droop characteristics are drawn in the middle of Fig. 2 to respond to reactive demands from the two microgrids. Their mathematical representations are given in (6), shown at the bottom of the page, where σx is the interlinking reactive droop coefficient. Also found in (6) are subscripts IC, OL, and HOL for representing the interlinking converter, overloaded threshold, and heavily overloaded threshold, respectively. As intended, (6) gives rise to zero reactive power command for interlinking converter “x” when its detected terminal voltage for microgrid “x” lies in the UL range. The interlinking converter can then be turned off if its active power command, discussed in the next section, is also zero. Upon the increase of the reactive loading in microgrid “x,” its accompanied terminal voltage detected by interlinking converter “x” falls into the OL range. Reactive support from the interlinking converter then gradually increases along the usual linear droop characteristic until it saturates at a value marked by Qx,IC,max in the HOL range. Limit Qx,IC,max can be set equal to the interlinking converter rating Sx,IC,max if active power transfer during heavy reactive loading is not intended. The characteristics described earlier can, in principle, operate independently since reactive powers generated by interlinking converters “a” and “b” need not be balanced. It is therefore possible to have both interlinking converters turned off, one of them turned on, or both of them turned on. The second scenario is, however, not possible with active power transfer, where power balance must be maintained at both terminals of the interlinking converters. It can therefore be more efficient if the interlinking converters are designed to focus more on reactive generation. B. Interlinking Active Droop Characteristics One of the reasons for tying microgrids together is to share their active power capacities so that backup generators need not be kept as reserves. For that, a mechanism for controlling the interlinking converters is required, which, as per reactive power control, is effected by two interlinking active droop characteristics shown in the middle of Fig. 2. Mathematically, they can be represented by (7) shown at the bottom of the next page, where ρx is the interlinking active droop coefficient. is the active power requested by microgrid “x,” In (7), Px,IC which should not be confused with the actual active power transferred by the interlinking converters. is determined by the detected frequency The value for Px,IC in microgrid “x,” which, when in the UL range, gives rise to zero active power requested by the self-sufficient microgrid “x.” Its value increases only when the detected frequency enters the OL range, before saturating at Px,IC,max in the HOL range. The value for Px,IC,max should be fixed according to (7), whose
Vx,OL < Vx ≤ Vx,max → Underloaded, Vx,HOL < Vx ≤ Vx,OL → Overloaded, Vx,min < Vx ≤ Vx,HOL → Heavily Overloaded,
x = a or b
(6)
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reasoning can be explained with the example of microgrid “a” requesting for Pa,IC,max from microgrid “b.” The maximum amount that can be provided by microgrid “b” is no doubt its rated active value Pb,max if its own loading is at zero. Therefore, Pa,IC,max should be set equal to Pb,max (or lower) with the same reasoning applicable to Pb,IC,max . should not be immediThe requested active power Px,IC ately supplied by interlinking converter “x” because of the requirement to maintain power balance at the terminals of the two interlinking converters. It is therefore important for the interlinking droop scheme to consider power requests from both microgrids before deciding on the actual active amount to transfer. One expression that can perform the necessary is given in (8), shown at the bottom of the page, which, when implemented, will give rise to the following intended results. 1) Actual active powers transferred by the interlinking con∗ ∗ and Pb,IC are always equal, but with oppoverters Pa,IC site polarities when in the steady state. 2) Their values are zero when both microgrids are UL and hence do not need additional active supports. Their values are again zero when both microgrids are HOL and hence do not have extra generation capacities for sharing. The interlinking converters can thus be turned off if their respective reactive generations are also at zero. 3) Their values are nonzero under other conditions. The cap is at microgrid “b” rating Pb,max if power is transferred from microgrid “b” to “a.” Similarly, for the reverse flow of power from microgrid “a” to “b,” the power transfer is capped at Pa,max . The previously described active power transfer works fine with large enough interlinking converter ratings. It must, however, be modified slightly if smaller rated interlinking converters are preferred. The recommended modification is shown in (9) at the bottom of the page, where the original active power ∗ has been compared with the remaining intertransfer Px,IC
Px,IC
⎧ ⎨ 0, = ρx (fx − fx,OL ), ⎩ Px,IC,max ,
Pa,IC,max
∗∗ Pa,IC
∗∗ = −Pb,IC
⎧ ⎪ ⎨ =
sign
⎪ ⎩P∗
∗ Pa,IC
a,IC ,
Pb,IC
Pb,IC,max Pb,IC Pb,IC,max
×min
C. Overall Block Diagram The block diagram showing the realization of the aforementioned interlinking droop control can be found in Fig. 4, which comparatively is different from the individual microgrid source control shown in Fig. 3. The former measures voltage magnitude and frequency, usually through a phase-locked loop, before generating the required active and reactive power references for the interlinking converters. The latter does the reverse of measuring voltages and currents for computing active and reactive powers, before using them to generate the required voltage and frequency references for the source. Their implementations and theoretical backgrounds are therefore different even though they are based on the same droop principles of active power versus frequency and reactive power versus voltage magnitude. Aside from the aforementioned observations, it can be seen in Fig. 4 that an additional activation trigger has been introduced. The thought is to activate the interlinking converter operation only when its active and reactive generations rise above some specified thresholds Px,act and Qx,act , which can be fractional multiples of the converter rating (e.g., 25% of Sx,IC,max ). This can, in principle, further minimize losses since it is usually not efficient to operate the converter at such low power value. IV. E XPERIMENTAL R ESULTS To verify the integrated performances of the microgrid and interlinking droop schemes, an experimental setup consisting of four six-switch converters was built. Two converters were for
fx,OL < fx ≤ fx,max → Underloaded, fx,HOL < fx ≤ fx,OL → Overloaded, fx,min ≤ fx ≤ fx,HOL → Heavily Overloaded,
⎧ P a,IC ⎨ Pa,IC,max − = P a,IC ⎩ −
∗ ∗ Pa,IC = −Pb,IC
linking converter capacities after accounting for their reactive ∗ is larger, a modified actual power power generations. If Px,IC ∗∗ reference Px,IC , capped at the lowest remaining capacity, will be used instead to avoid stressing the converters improperly.
2 Sa,IC
−
× Pa,IC,max ,
× Pb,IC,max ,
Q∗a,IC
if positive if negative
⇒
Pa,IC,max = Pb,max Pb,IC,max = Pa,max
∗ −Pa,max ≤ Pa,IC ≤ Pb,max ∗ −Pb,max ≤ Pb,IC ≤ Pa,max
2 2 2 ∗ , Sb,IC − Qb,IC ,
(7)
(8)
∗ in magnitude if smaller thanPa,IC otherwise (9)
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Fig. 4.
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Realization of interlinking droop control scheme.
emulating microgrids “a” and “b” (see Fig. 3), while the other two were for forming the back-to-back interlinking converters (see Fig. 4). The converters were digitally controlled with their respective active power, reactive power, frequency, and voltage ranges listed in Table I. These ranges were chosen to satisfy the usual tradeoff between regulation and sharing accuracy of any droop controller with no other consideration needed. The experiment was performed with loading conditions in the two microgrids arbitrarily adjusted to give three different cases consecutively. Voltages and frequencies measured by the interlinking converters for these three cases were summarized in Table II, whose corresponding loading classifications were also indicated (UL, OL, or HOL). Based on these classifications, different decisions on power generation were effected for the three cases, whose details are discussed as follows.
TABLE I E XPERIMENTAL D ROOP PARAMETERS AND R ANGES
A. Case 1 Microgrid “a,” classified as OL for both active and reactive generations, had requested for active and reactive supports from interlinking converter “a.” Microgrid “b,” on the other hand, had requested for reactive power only from interlinking converter “b” to reinforce its OL reactive generation. Being independent, the requested reactive powers were generated by the interlinking converters immediately, whose values were read from either the “0 to 60 s” interval of Fig. 5 or the third column of Table III. This was, however, not the case for the requested active powers, whose values must jointly be substituted into (8) and (9) to get the actual active powers generated by the interlinking converters. Read from the “0 to 60 s” interval of Fig. 5 or the third column of Table III, their values were Pa,IC = 0.27 kW and Pb,IC = −0.27 kW, representing a transfer of active power from the UL microgrid “b” to OL microgrid “a.” Being of equal magnitude also guaranteed power balance with no variation of dc-link capacitor voltage observed.
B. Case 2 The loading conditions were changed to case 2, whose reactive classifications caused interlinking converter “a” to generate
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TABLE II VOLTAGES AND F REQUENCIES D ETECTED BY I NTERLINKING C ONVERTERS U NDER D IFFERENT O PERATING C ONDITIONS
Fig. 5. Experimental actual active and reactive power generations. TABLE III ACTUAL ACTIVE AND R EACTIVE P OWERS G ENERATED U NDER D IFFERENT O PERATING C ONDITIONS
TABLE IV A DJUSTMENT OF M AXIMUM ACTIVE AND R EACTIVE P OWER VALUES U NDER D IFFERENT O PERATING C ONDITIONS
no reactive power and interlinking converter “b” to generate 0.75 kVAr. Its active classifications also led to two requested active powers, whose values were close. They gave rise to actual active power values, whose magnitudes were smaller than the activation thresholds (see Section III-C), hence causing the interlinking converters to turn off if they were also not producing reactive powers. From the values read from the “60 to 160 s” interval of Fig. 5 or the fourth column of Table III, interlinking converter “a” with no active and reactive generation was, in fact, turned off to minimize losses during the experiment. The same could not be effected on interlinking converter “b”, whose reactive generation was nonzero.
C. Case 3 The loading conditions were finally changed to case 3, where both microgrids were UL in terms of reactive generation but OL in terms of active generation. The reactive powers generated by the interlinking converters were therefore zero. Their requested active powers were, however, nonzero but equal, which, when substituted into (8) and (9), gave rise to zero actual active powers transferred by the interlinking converters. These values could be read from the “160 to 200 s” interval of Fig. 5 or the last column of Table IV. Since the active and reactive power generations here were all zero, both interlinking converters were turned off to minimize losses, as intended.
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Fig. 6.
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Experimental maximum active and reactive power adjustments.
D. Adaptively Adjusted Power Limits As explained in Section II, maximum reactive power Qx,max that could be produced by each microgrid was constantly changing with its active power generation Px [see (4)]. To illustrate the variation, values for Qx,max and Px were plotted on the left and right of Fig. 6 for the two microgrids. Their values were also tabulated in the third and last rows of Table IV for easier reference. Aside from this variation, the variation experienced by the interlinking converter when computing its remaining capacity using (9) could also be plotted. The variation was shown in the middle of Fig. 6, which, for convenience of notating, was 2 − (Q∗x,IC )2 . Relevant values read labeled as Px,rem − Sx,IC from the plots were also tabulated in the fourth and fifth rows of Table IV.
V. C ONCLUSION This paper presents distributed power management schemes for intertied microgrids based on the droop operating principles. Unlike a single microgrid, sources within the intertied microgrids can concentrate more on harnessing active power, leaving most of the reactive generation to the interlinking converters. Where necessary, the interlinking converters can also transfer active power from an UL microgrid to an OL microgrid, which hence does not need to keep backup generators. Such transfer stops when both microgrids do not need additional generation or do not have extra generating capacities for sharing. The interlinking converters can then be turned off to reduce its operating losses. These discussed features are attained autonomously based only on locally measured variables. Their effectiveness has already been tested in experiment. R EFERENCES [1] H. Farhangi, “The path of the smart grid,” IEEE Power Energy Mag., vol. 8, no. 1, pp. 18–28, Jan./Feb. 2010. [2] M. G. Simoes, R. Roche, E. Kyriakides, S. Suryanarayanan, B. Blunier, K. D. McBee, P. H. Nguyen, and A. Miraoui, “A comparison of smart grid technologies and progresses in Europe and the U.S.,” IEEE Trans. Ind. Appl, vol. 48, no. 4, pp. 1154–1162, Jul./Aug. 2012. [3] S. Massoud Amin and B. F. Wollenberg, “Toward a smart grid: Power delivery for the 21st century,” IEEE Power Energy Mag., vol. 3, no. 5, pp. 34–41, Sep./Oct. 2005.
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Inam Ullah Nutkani (M’08) received the B.E. degree in electrical engineering from the NED University of Engineering and Technology, Karachi, Pakistan, in 2003 and the M.S. degree in power engineering from Nanyang Technological University, Singapore, in 2007. He is currently pursuing a part-time Ph.D. degree at Nanyang Technological University. After receiving the B.E. degree, he worked as an Assistant Executive Engineer with KESC Karachi, an electric utility company, from 2003 to 2004, and as an Assistant Manager with NESCOM, Islamabad, Pakistan, a national R&D organization of Pakistan, from 2004 to 2006. After receiving the M.S. degree, his employment experience has included working as a Technology Development Engineer with West Energy Singapore and a Senior Design Engineer with JM Pang & Seah, Singapore, a professional electrical and mechanical consulting firm. From 2008 to 2012, he was a Research Engineer with the Experimental Power Grid Centre, A*STAR, Singapore, a national R&D agency of Singapore, and since 2012, he has been a Senior Research Engineer at the same company. At EPGC, he is working on industry R&D projects in the area of fault current limiters, renewable penetration impact on grid, LFO impact on WTG, and microgrid operation. During the summer of 2009, he was a Visiting Researcher with the CSIRO, Newcastle, Australia, for joint research experiments. His fields of interest include power electronic applications for distribution networks and microgrids, renewable integration, superconducting and nonsuperconducting fault current limiters, dc microgrids, autonomous and interlinked ac microgrid design, control, and management.
Poh Chiang Loh (S’01–M’04–SM’12) received the B.Eng.(Hons.) and M.Eng. degrees in electrical engineering from the National University of Singapore, Singapore, in 1998 and 2000, respectively, and the Ph.D. degree in electrical engineering from Monash University, Clayton, Australia, in 2002. During the summer of 2001, he was a Visiting Scholar with the Wisconsin Electric Machine and Power Electronics Consortium, University of Wisconsin, Madison, WI, USA, where he worked on multilevel inverters and their modulation. From 2002 to 2003, he was a Project Engineer with the Defence Science and Technology Agency, Singapore, managing defense infrastructure projects and exploring technology for defense applications. From 2003 to 2009, he was an Assistant Professor at Nanyang Technological University, Singapore, where he has been an Associate Professor since 2009. In 2005, he was a visiting staff member, first at the University of Hong Kong, China, and then at Aalborg University, Aalborg, Denmark. In 2007 and 2009, he again returned to Aalborg University, first as a visiting staff member working on matrix converters and the control of grid-interfaced inverters and then as a guest member of the Vestas Power Program. Dr. Loh has received three paper prizes in total from the IEEE in 2003, 2006, and 2012. He is currently serving as an Associate Editor of the IEEE T RANSACTIONS ON P OWER E LECTRONICS and IEEE T RANSACTIONS ON I NDUSTRY A PPLICATIONS.
Frede Blaabjerg (S’86–M’88–SM’97–F’03) received the Ph.D. degree from Aalborg University, Aalborg, Denmark, in 1995. He was with ABB-Scandia, Randers, from 1987 to 1988. He became an Assistant Professor with Aalborg University in 1992, where he became an Associate Professor in 1996 and a Full Professor of power electronics and drives in 1998. He was a part-time Research Leader for wind turbines at the Research Center Risoe. In 2006–2010, he was the Dean of the Faculty of Engineering, Science and Medicine and became a Visiting Professor at Zhejiang University, Hangzhou, China, in 2009. His research areas are in power electronics and applications such as wind turbines, PV systems, and adjustable-speed drives. Prof. Blaabjerg was the Editor in Chief of the IEEE T RANSACTIONS ON P OWER E LECTRONICS during 2006–2012. He was a Distinguished Lecturer for the IEEE Power Electronics Society in 2005–2007 and for the IEEE Industry Applications Society from 2010 to 2011. He was the Chairman of EPE’2007 and PEDG’2012—both held in Aalborg. He was the recipient of the 1995 Angelos Award for his contribution to modulation technique and the Annual Teacher Prize from Aalborg University. He was also the recipient of the Outstanding Young Power Electronics Engineer Award from the IEEE Power Electronics Society in 1998. He was the recipient of 13 IEEE Prize Paper Awards and another prize paper award at PELINCEC Poland 2005. He was also the recipient of the IEEE PELS Distinguished Service Award in 2009 and the EPE-PEMC 2010 Council Award. Additionally, he was the recipient of a number of major research awards in Denmark.
