IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 50, NO. 5, SEPTEMBER/OCTOBER 2014 3549 Validation of the Plug-and-Play AC/AC Power Electronics Building Block (AC-PEBB) for Medium-Voltage Grid Control Applications Amrit R. Iyer, Student Member, IEEE, Rajendra Prasad Kandula, Student Member, IEEE, Rohit Moghe, Member, IEEE, Jorge E. Hernandez, Student Member, IEEE, Frank C. Lambert, Senior Member, IEEE, and Deepak Divan, Fellow, IEEE Abstract—As ac-to-ac power conversion becomes increasingly important in applications such as grid power control and motor control, demand for a plug-and-play converter has increased. Traditional voltage-source-inverter-based ac-to-ac power converters such as the back-to-back converter require dc-link capacitors that limit their reliability or significantly increase their size and cost. This paper examines a plug-and-play concept for achieving ac-to-ac power conversion via the direct-ac/ac power electronic building block (AC-PEBB). The AC-PEBB is a compact selfcontained cell requiring no intermediate energy storage. It can be used in a variety of ac/ac power electronic applications such as matrix converters and controllable network transformers. This paper details the design of an 800-V 100-A AC-PEBB prototype to be used in a 13-kV 1-MVA application. Successful test results using several AC-PEBBs in a variety of configurations are demonstrated at up to 11 kV and 600 kVA. Index Terms—AC–AC power conversion, flexible ac transmission systems, high-voltage techniques, matrix converters, phase control, power control, power semiconductor switches, snubbers. I. I NTRODUCTION A C-TO-AC power conversion is becoming important in many utility and industrial applications. The utility grid is a prime example, where a dramatic increase in the level of renewable resources connected to the grid is occurring in order to increase energy security and reduce carbon emissions. However, the spatial and temporal variability of new wind and solar generation is poorly correlated with existing loads, leading to the need for significant transmission investments in order to reduce possible congestion [1], [2]. In lieu of transmission improvements, which are typically difficult and time consuming to negotiate and implement, dynamic grid power control can be used to enhance system utilization [2]–[5]. Important metrics Manuscript received October 29, 2013; accepted January 23, 2014. Date of publication February 7, 2014; date of current version September 16, 2014. Paper 2013-PEDCC-715, presented at the 2013 IEEE Energy Conversion Congress and Exposition, Denver, CO, USA, September 16–20, and approved for publication in the IEEE T RANSACTIONS ON I NDUSTRY A PPLICATIONS by the Power Electronic Devices and Components Committee of the IEEE Industry Applications Society. This work was supported by the United States Department of Energy through the Advanced Research Projects Agency—Energy program. The authors are with the School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332 USA (e-mail: ariyer@ gatech.edu; krprasad@gatech.edu; rmoghe@varentec.com; jorge.hernandez@ gatech.edu; frank.lambert@neetrac.gatech.edu; deepak.divan@ece.gatech. edu). 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.2014.2304583 for power controllers to be used for transmission applications include high reliability, scalability to 345 kV and 200 MW, and attractive cost. Traditional solutions for grid power control, such as high-voltage dc (HVDC) lines and flexible ac transmission system (FACTS) devices, suffer from high cost [2], [6], high complexity [6], relatively high losses [2], [5], and relatively low reliability; typical HVDC and FACTS devices realize reliability levels of 95%–98% [7]–[9], whereas the US transmission network operates at levels in excess of 99.9% [10]. To drive adoption of power controllers among utilities, a technology is needed that is lower in cost, higher in efficiency, easy to integrate, and does not lower single-point system reliability. One attractive solution that exists in the literature is the controllable network transformer (CNT), which adds a directac/ac fractionally rated power electronics converter to a load tap changer (LTC) transformer to achieve grid power control [11]. The fractional rating of the CNT allows for efficiencies greater than 99%, and its integrated fail-normal bypass switch allows for enhanced reliability. The design and initial testing of an ac/ac power electronic building block (AC-PEBB) for a CNT application was previously outlined by Iyer et al. in [12]. This paper expands upon the AC-PEBB in [12], where a compact, modular, and scalable direct-ac/ac building block is examined. Important design considerations of the AC-PEBB are explained, and experimental results are shown for a CNT application at 11 kV and 600 kVA using six AC-PEBB cells in a variety of configurations. II. S CALING THE CNT W ITH AC-PEBB C ELLS A. CNT Review For a CNT with a tap ratio of 1 to n and a virtual quadrature source (VQS)-based duty cycle of D = K0 + K2 sin(2ωt + Φ) (1) active and reactive power can be controlled in a two-bus system, such as in Fig. 1, according to (2) and (3), as shown in [11], i.e., V 2B V 2A sin δ − cos(δ + Φ) ωL ωL 1 + n − 2K0 n nK2 with A = , B= (2) 1− n2 1 − n2 BV 2 V2 2 AV 2 cos δ+ sin δ. (3) Qout = A2 − B 2 − ωL 3 ωL ωL Pout = 0093-9994 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. 3550 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 50, NO. 5, SEPTEMBER/OCTOBER 2014 Fig. 1. Conceptual one-line diagram of CNT connecting two buses. Fig. 2. cell. Fig. 3. Series stacking of AC-PEBBs in a CNT configuration. Fig. 4. Parallel stacking of AC-PEBBs in a CNT configuration. (a) Schematic of the AC-PEBB cell. (b) Solid model of the AC-PEBB B. AC-PEBB Overview The PEBB concept, explained in [13], outlines a vision for a singular building block that can reduce the size and cost of power electronics, while facilitating easier installation, maintenance, and manufacturing. PEBB technology has been proven useful in transmission-level VSC applications, where the modular multilevel converters (MMC or M2 C) have been cascaded for use in HVDC power transmission [14]. For directac/ac converters, an ac PEBB design has been proposed in [15]; however, the topic of scaling to higher voltages for direct-ac PEBBs has yet to be addressed. The AC-PEBB described in this paper and shown in Fig. 2 is a self-contained unit that provides a plug-and-play functionality for ac-to-ac power conversion applications. It provides a compact modular building block for direct-ac/ac applications and allows series and parallel connection of cells to enable scaling to higher voltage and power levels. An AC-PEBB cell contains four insulated-gate bipolar transistors (IGBTs, or MOSFETs), forming two ac switches with a midpoint connection, as shown in Fig. 2(a). Each ACPEBB contains all necessary gate drive, snubber, and thermal management components, allowing the cell to be used as a standalone matrix or thin ac converter [16]. Through directac/ac conversion via the use of VQS as outlined in [17], large dc storage elements are not needed, allowing the cell to be only a few inches in size in each dimension, as shown in Fig. 2(b). Compactness, combined with the cell’s modularity, allows it to be easy to manufacture, install, and replace, thus reducing both upfront and operation and maintenance costs. cells. For example, Figs. 3 and 4 show various forms of a CNT prototype built by series and parallel connection of AC-PEBBs to the taps of a standard 1-MVA transformer. More advanced topologies are discussed in companion paper [18]. The major obstacles encountered when scaling the AC-PEBB to utility-scale power levels are ensuring equal voltage sharing between devices and providing protection from incorrect commutation sequences. These challenges can be overcome by the active snubber concept proposed in [19]. The half-wave rectified sources connected through diodes across each device in Fig. 2(a) are conceptual representations of the active snubber technology being used in the AC-PEBB. III. R EVIEW OF THE AC-PEBB C. Scaling the AC-PEBB Usage of the AC-PEBB at higher voltage and power levels is accomplished via series and parallel connection of multiple The AC-PEBB consists of five parts: the main switching devices, the gate drivers, the snubber circuits, the bus bars, and the thermal management system. IYER et al.: VALIDATION OF THE PLUG-AND-PLAY AC-PEBB FOR GRID CONTROL APPLICATIONS 3551 Fig. 6. Hybrid snubber circuit for AC-PEBB. Fig. 5. AC-PEBB snubber board. Fig. 7. Active snubber circuit and buck–boost control for AC-PEBB. A. Main Device Selection The main AC-PEBB switching devices are GeneSiC Semiconductor’s GA100XCP12-227. These are 1200-V 100-A silicon IGBTs with a built-in silicon carbide antiparallel diode. The SiC diodes contained in this device have minimal reverse recovery and high temperature tolerances, allowing device operation at tens of kilohertz and temperatures as high as 150 ◦ C. The device’s 1200-V rating allows it to safely operate at voltages with a peak of up to 800 V, corresponding to a root mean square (RMS) voltage of 565 V; a 400-V buffer is in place to account for voltage spikes during switching due to stray inductances. An AC-PEBB built with these devices is capable of acting as a 56.5-kVA standalone converter. Additionally, as AC-PEBBs can operate as thin ac converters, to provide full power flow control, they only need to be rated at about 10%–20% of the level of the asset to be augmented, depending on the controllability range needed. Therefore, a single AC-PEBB composed of these GeneSiC devices can be used to route power flows of as high as 565 kVA. Accounting for the small size of GeneSiC’s devices, which are 1.3 × 1.6 inches, series and parallel stacking of GeneSiC-based AC-PEBBs has the ability to reach transmission level powers in a compact size. B. Snubber Board Design The AC-PEBB snubber board, as shown in Fig. 5, consists of four snubber circuits organized on a single PCB, which is directly screwed onto the IGBTs of the AC-PEBB. A hybrid passive and active snubber approach is used, as shown in Fig. 6. The passive snubber portion of the design protects the converter during a system-level fault until external circuit breakers, relays, or fuses have a chance to react. The active snubber portion of the hybrid snubber, as outlined in [19], creates a half-wave rectified envelope around each IGBT, providing protection from internal faults, such as incorrect commutations near zero crossings, and ensuring that equal voltage is seen across every device. A modified version of the buck–boost design in [19] is employed in the AC-PEBB to control each active snubber and is schematically shown in Fig. 7. More information on the electrical design of the snubber circuit for an 800-V 100-A application can be seen in [20]. Any stray inductance in the snubber-to-IGBT path leads to voltage spikes during snubber operation, and because the snubber operates during incorrect commutations or fault conditions, large currents and di/dt’s will be experienced during these situations. Therefore, parasitic inductances significantly reduce the effectiveness of the snubber circuit in limiting voltage spikes. The snubber board design in Fig. 5 places the snubber capacitor (CS ), diode (DS ), and varistor (MOVS ) as close as possible to the IGBTs they are protecting, thus minimizing the inductance in the snubber path and maximizing their effectiveness. The control circuitry for the snubber voltage envelope creation is moved onto the gate drive board to reduce the overall size of the AC-PEBB. C. Gate Drive Board Design The gate drivers for the AC-PEBB are chosen to be Powerex VLA-502 modules mounted onto modified Powerex BG2A gate drive boards. The BG2A boards are augmented with protection and fault management circuitry, control circuitry for the ACPEBB’s snubber circuits, and fiber optic connections to carry all signals to and from the converter’s main microcontroller unit. If long signal wires are employed in lieu of fiber optic connections, large amounts of switching noise induced by the converter’s extremely high di/dt levels would compromise the integrity of the digital control signals, even if a twisted-pair configuration is used. Finally, gate drive board is designed to directly plug into the main snubber board using low-inductance bus-bar connections in order to minimize inductance in the gate–emitter path of each IGBT. Fig. 8 shows the final gate drive board as well as the layout of its various subcircuits. Each board controls two IGBTs. 3552 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 50, NO. 5, SEPTEMBER/OCTOBER 2014 TABLE I AC-PEBB S PECIFICATION S UMMARY Fig. 8. Gate drive board for AC-PEBB. Fig. 10. PTMS of 5 kW to 40 ◦ C for up to 12 AC-PEBB cells. With the integration of the PTMS into the AC-PEBB, the cell is able to independently operate from electrical, thermal, and mechanical points of view. Additionally, the PTMS removes the need for fans, pumps, and other moving parts, increasing the expected lifetime of the converter to 30 years. This lifetime puts the AC-PEBB alongside most other utility assets in terms of robustness, increasing its commercial appeal and ease of integration. IV. AC-PEBB-BASED 1-MW CNT D ESIGN A. 1-MW Test Bed Design Fig. 9. Complete AC-PEBB cell. D. Completed Cell Specifications The completed AC-PEBB, mounted on a heat sink, is shown in Fig. 9. Again, inductance was kept to a minimum through the use of wide bus bars and a compact design. Spacers and 30-mil Nomex insulation are present to keep the various layers of the AC-PEBB electrically isolated. The entire cell is a cube that is approximately 6 × 6 × 10 inches in size. The specifications of the designed AC-PEBB are summarized in Table I. E. Thermal Management For higher power applications, cooling is provided by a novel passive thermal management system (PTMS), shown in Fig. 10, that is capable of supporting up to 12 cells. The PTMS involves a main AC-PEBB heat sink feeding into an oil tank and fin array. The losses generated by the system are used to create a thermosiphon to circulate the oil between the main AC-PEBB heat sink and external fin array [21]. The AC-PEBB-based CNT laboratory prototype consists of two 13-kV primary, 1300-V/650-V secondary, 167-kVA transformers, whose primary and secondary windings are series connected, and a set of line inductors, as shown in Figs. 11 and 12. The two transformer sets are representative of two buses in an arbitrary power system. A set of AC-PEBBs is connected across one of the transformers between its +/−650-V taps, achieving a CNT configuration with a tap ratio of n = 9.5%. Current can be circulated between the system’s transformers without power transfer occurring with the grid. Using a POWERSTAT, voltage levels on the system’s input and output voltage buses are scalable from 0 to 13650 V (RMS). For safety reasons and to limit the voltage seen by the converter’s PTMS, the center tap of the transformer of bus 1 was used as ground. Without the AC-PEBB-based converter, both bus 1 and bus 2 have the same voltage magnitude and phase, and there is no power transfer across the line. With the AC-PEBB-based converter, the effective voltage of bus 1 is controlled to be different from bus 2, in magnitude or phase or both, to induce energy flow on the line. Hence, the functionality of the CNT and AC-PEBBs can be demonstrated by controlling the power between the two buses. IYER et al.: VALIDATION OF THE PLUG-AND-PLAY AC-PEBB FOR GRID CONTROL APPLICATIONS 3553 Fig. 11. Two-bus system of 13 kV with an input-side LTC transformer, of tap ratio n = 9.5%, connected in a CNT configuration. Fig. 12. Two-bus system of 13 kV, 1 MW and AC-PEBB/CNT test bed. B. AC-PEBB Control Board Design Fig. 13 shows the control flow diagram for the AC-PEBB cell-based CNT. Switching signals originating from a digital signal processor (DSP) are sent to an electrically erasable programmable read-only memory (EEPROM), which keeps track of the state of the system and its switching devices. The EEPROM is preprogrammed with switching sequences that ensure proper ac commutation of the converter and prevent any short circuiting of the source or open circuiting of the line inductor current; two-level, three-level, voltage, and currentbased commutation sequences have all been implemented (for more information on multilevel commutation, refer to companion paper [18]). The EEPROM switching signals are sent to the converter’s gate driver circuits. Converter status, fault status, and switch voltage and line current measurements, used in voltage and current commutation algorithms by the DSP, are then collected from the converter, conditioned, and sent back to the DSP. Communication of signals between the DSP control and the AC-PEBB occurs via fiber optic links. V. E XPERIMENTAL R ESULTS A. Parallel Operation of Modules With six AC-PEBB cells configured as two parallel, threelevel, direct-ac/ac, the CNT was shown to be able to control 600 kVA of power at a bus voltage of 8 kV and line current of 3554 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 50, NO. 5, SEPTEMBER/OCTOBER 2014 Fig. 15. Six AC-PEBB cells connected as dual three-level converters, as shown in Fig. 14, and mounted to a custom PTMS. Fig. 13. Control flow for direct-ac/ac converter and CNT comprised of up to 12 AC-PEBB cells. Fig. 16. Real power demonstration of an 8-kV, 75-A, 600-kVA two-bus system with a three-level CNT. Fig. 14. Parallel three-level direct-ac/ac converter system made up of six AC-PEBB cells. 75 A. Fig. 14 shows the schematic representation of the ACPEBB layout, with companion paper [18] providing more details on the three-level converter topology. Fig. 15 shows the test setup with six AC-PEBB cells mounted on the PTMS and connected to the two-bus system. Figs. 16–19 demonstrate the CNT’s ability to have complete control over the phase angle of the line current, showing correct operation of the AC-PEBB cells. In those plots, the 8-kV (RMS) Fig. 17. Negative-real power demonstration of an 8-kV, 75-A, 600-kVA twobus system with a three-level CNT. line voltage is the uppermost waveform, followed by the 75-A (RMS) line current. In Fig. 16, these two waveforms are in phase; in Fig. 17, they are 180◦ out of phase; in Fig. 18, the current is lagging the voltage by 90◦ ; and in Fig. 19, the current is leading by 90◦ . This variation of phase was done in real time; thus, full power control is seen with operation shown in positive-real, negative-real, lagging-reactive, and leadingreactive power flow modes at 600 kVA. Power magnitude control was also demonstrated, although not shown. IYER et al.: VALIDATION OF THE PLUG-AND-PLAY AC-PEBB FOR GRID CONTROL APPLICATIONS 3555 Fig. 18. Lagging-reactive power demonstration of an 8-kV, 75-A, 600-kVA two-bus system with a three-level CNT. Fig. 20. Frequency spectrum of 8-kV, 75-A, 600-kVA bus voltage in a twobus system with a three-level CNT. stages shown in Fig. 11. The filter capacitors are necessary to prevent the 25-kHz electromagnetic interference caused by the CNT from propagating back to the source. Through experimental data gathered by varying the size of the filter capacitors, the equivalent source inductance was calculated to be initially 30 mH from the following equation: 1 . (4) Lsource = (3ω0 )2 C2F Fig. 19. Leading-reactive power demonstration of an 8-kV, 75-A, 600-kVA two-bus system with a three-level CNT. The third and fourth waveforms in Figs. 16–19 show the synthesized converter output voltage and the voltage across each of the two parallel AC-PEBB-based converters. These two waveforms depict the 25-kHz VQS-based PWM technique used in this experiment and show the parallel modules successfully operating in unison. B. Issues With Harmonics and Resonances Note the third-harmonic ripple seen in the waveforms in Figs. 16–19. This is a byproduct of the VQS modulation method, as explained in [17], and would need to be filtered out in a practical application, as demonstrated in [22]. As shown in Fig. 20, a third harmonic at 41% of the fundamental is seen on converter and bus voltages in addition to the current. This phenomenon was not noticeable at the lower power level tests performed in [12] and is due to the voltage drop across the impedance of the source and power transformers from the thirdharmonic current. Filtering of the third-harmonic current as in [22] will also solve the voltage harmonic issue. In addition to harmonics, resonances at the third harmonic were experienced. These resonances were found to be occurring between the filter capacitors, i.e., CF , and input transformer By lowering the number of input transformer stages, the tap ratio of the bus transformers (from 17% to 9.5%), and the amount of filter capacitance (from 200 to 50 μF), the resonant frequency of the system was able to be moved far enough away from 60 Hz to eliminate interference with system operation; without these steps, the source current drawn at higher power levels due to resonances would have overloaded the POWERSTAT and system transformers. Higher order (5th through 11th) harmonics were also seen in the system, caused by the filter capacitors interacting with various smaller parasitic elements present in the system. While smaller in magnitude, these harmonics increased the number of zero-crossings for voltage and current under certain operating conditions; spending more time around zero-crossings resulted in more missed commutations. To damp higher order harmonics, resistors RF were placed in series with the filter capacitance values. Fig. 11 shows the final configuration used for higher current and higher power testing. C. Converter Efficiency With the AC-PEBB configuration in Fig. 14, the input and output voltages and currents of the AC-PEBB system were measured in order to estimate system efficiency. Table II shows the results of this measurement at 400 kVA, where the CNT is over 99% efficient, neglecting transformer core and winding losses. This high efficiency is mainly due to the converter’s ability to gain full control range without being exposed to the full bus voltage. 3556 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 50, NO. 5, SEPTEMBER/OCTOBER 2014 TABLE II CNT C ONVERTER E FFICIENCY AT 6.7 kV, 60 A, 400 kVA Fig. 22. Demonstration of voltage sharing between individual IGBTs in a CNT at 5 kV, 10 A, and 50 kVA. Fig. 21. Real power demonstration of an 11-kV, 15-A, 165-kVA two-bus system with a three-level CNT. D. High-Voltage Testing Testing was performed at lower current and higher voltage levels to ensure that a single AC-PEBB was able to handle its full voltage rating of 565 V (RMS), 800 V (peak). Fig. 21 shows these results at 11 kV, 15 A, and 165 kVA. The fourth waveform in this figure shows an individual IGBT voltage peaking at 750 V, demonstrating successful operation of the AC-PEBB at near its rated value of 800 V. Note that at higher voltages and lower currents, the third-harmonic currents do not significantly affect bus voltage, as explained in Section V-B. E. Series Operation The series-stacking capability of AC-PEBBs was examined by validating their voltage-sharing capabilities. Fig. 22 shows successful voltage sharing at a 5-kV (RMS) bus voltage, where 770 V (peak) is impressed across each AC-PEBB. The purple waveform shows an individual device voltage being held to about 335 V (peak) by the green waveform, which is the active snubber voltage and exactly half of the voltage across an ACPEBB cell, demonstrating equal voltage sharing and seriesstacking capabilities of the cell. VI. C ONCLUSION This paper has outlined the construction and testing of an AC-PEBB unit built for operation at MV levels. Important design considerations were highlighted, and viability of the concept has been shown successful experimental results at up to 11-kV and 600-kVA levels and low-voltage prototyping. The PEBB proved to be compact, easy to interconnect, and highly efficient. Practical issues relating to medium-voltage ac/ac converter design were also examined and uncovered during the course of this work. Harmonic and resonance issues showed themselves to be significantly worse at higher power levels and will be a major challenge in any practical MV or HV application. Additionally, series stacking of cells became difficult as the tuning of each analog active snubber circuit was cumbersome when many cells were used. A more robust active snubber control design is recommended for true plug-and-play capability to be achieved. ACKNOWLEDGMENT The authors would like to thank Dr. J. Rhett Mayor, B. Loeffler, and A. Semidey of the Department of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA, USA, for their contributions to this project relating to the construction and thermal management of the CNTs used in this research. The authors would also like to thank T. Parker and the engineers/technicians of the National Electric Energy Testing Research and Applications Center for their assistance in the construction and operation of the medium-voltage test bed used in this research. R EFERENCES [1] “National electric transmission congestion study,” U.S. Department of Energy, Washington DC, USA, Dec. 2009. [2] P. Fairley, Germany Takes the Lead in HVDC, IEEE Spectrum, New York, NY, USA. (2013, Apr. 29). [Online]. Available: http://spectrum.ieee.org/ energy/renewables/germany-takes-the-lead-in-hvdc [3] F. Kreikebaum, D. Das, and D. Divan, “Reducing transmission investment to meet renewable portfolio standards using controlled energy flows,” in Proc. ISGT, Jan. 19–21, 2010, pp. 1–8. [4] Y. Xiao, Y. Song, L. Chen-Ching, and Y. Sun, “Available transfer capability enhancement using FACTS devices,” IEEE Trans. Power Syst., vol. 18, no. 1, pp. 305–312, Feb. 2003. [5] P. Fairley, Flexible AC Transmission: The FACTS Machine, New York, NY, USA. 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Divan, J. Sastry, A. Prasai, and H. Johal, “Thin AC converters—A new approach for making existing grid assets smart and controllable,” in Proc. IEEE PESC, Jun. 15–19, 2008, pp. 1695–1701. [17] D. Divan and J. Sastry, “Voltage synthesis using dual virtual quadrature sources—A New Concept in AC Power Conversion,” in Proc. IEEE PESC, Jun. 2007, pp. 2678–2684. [18] A. Iyer, R. Kandula, R. Moghe, F. Lambert, and D. Divan, “Scaling the controllable network transformer (CNT) to utility-level voltages with direct AC/AC power electronic building blocks (PEBBs),” in Proc. ECCE, Sep. 15–19, 2013, pp. 3049–3056. [19] A. Prasai and D. Divan, “Active AC snubber for direct AC/AC power converters,” in Proc. ECCE, Sep. 17–22, 2011, pp. 507–514. [20] A. Iyer, R. Moghe, R. Kandula, A. Prasai, and D. Divan, “Experimental validation of active snubber circuit for direct AC/AC converters,” in Proc. ECCE, Sep. 15–20, 2012, pp. 3856–3861. [21] B. H. Loeffler, “Modeling and optimization of a thermosiphon for passive thermal management systems,” M.S. thesis, Dept. Mech. Eng., Georgia Inst. Technol., Atlanta, GA, USA, 2012. [Online]. Available: https://smartech.gatech.edu/handle/1853/45960 [22] D. Das et al., “Design and testing of a medium voltage controllable network transformer prototype with an integrated hybrid active filter,” in Proc. IEEE ECCE, Sep. 17–22, 2011, pp. 4035–4042. Amrit R. Iyer (S’11) received the B.S.E.E. degree from the University of Illinois at UrbanaChampaign, Champaign, IL, USA, in 2003, with an emphasis in power electronics. He is currently working toward the Ph.D. degree in electrical engineering with the Electrical Energy Group, Georgia Institute of Technology, Atlanta, GA, USA, under Prof. D. Divan. He spent several semesters working with the Illinois Tiny Satellite Initiative, including serving as the lead for the group’s power management team. He has published over a dozen papers and has two patents pending in his areas of interest. His current research interests include power electronic converter design for medium- and high-voltage applications and smart sensing technologies for utility applications, where his work has been sponsored by General Electric, Advanced Research Projects Agency—Energy, the National Electric Energy Testing Research and Applications Center, and Georgia Tech’s Intelligent Power Infrastructure Consortium. Rajendra Prasad Kandula (S’10) received the B.Tech. degree from the National Institute of Technology, Nagpur, India, in 2002 and the M.Tech. degree from the Indian Institute of Science, Bangalore, India, in 2004. He is currently working toward the Ph.D. degree at the Georgia Institute of Technology, Atlanta, GA, USA. For three years, he was a Design Engineer with Bharat Heavy Electricals Ltd. R&D, New Delhi, India, working with high-power drives and gridconnected solar inverters. His research interests include power electronics for utility applications such as active filters and power routers for meshed systems. 3557 Rohit Moghe (S’07–M’12) received the B.Tech. degree in electrical engineering from the Indian Institute of Technology, Roorkee, India, in 2007, and the M.S. and Ph.D. degrees from the Georgia Institute of Technology, Atlanta, GA, USA, in 2010 and 2012, respectively. During May–August 2008 and 2009, he was a summer intern with the ABB U.S. Corporate Research Center and Siemens Energy & Automation, respectively. He is currently a Principal Engineer with Varentec Inc., San Jose, CA, USA. His research focuses on power electronic converters for utility applications such as VAR support, power flow controllers, medium-voltage solid-state transformers, and energy harvesting for wireless sensors. Dr. Moghe was awarded the Best Undergraduate Research Award by the Indian Institute of Technology. He was one of the founding members of the Energy Club at the Georgia Institute of Technology and served as President during 2010–2011. Jorge E. Hernandez (S’10) received the B.S. degree in electrical engineering from the Technological University of Panama, Panama City, Panama, in 2006 and the M.S. degree in electrical engineering from the Georgia Institute of Technology, Atlanta, GA, USA, in 2009. He is currently working toward the Ph.D. degree in the School of Electrical and Computer Engineering, Georgia Institute of Technology. From January to July 2007, he was a Research Engineer with the Technological University of Panama, overseeing renewable integration projects for rural communities. His research interests include power electronics for utility applications, and power system operation/control and electricity markets, under current practices and under scenarios with high wind and electric vehicle penetration. Frank C. Lambert (S’70–M’73–SM’87) received the B.S. and M.S. degrees in electrical engineering from the Georgia Institute of Technology, Atlanta, GA, USA. He is currently the Associate Director of the National Electric Energy Testing Research and Applications Center, Georgia Institute of Technology. He is responsible for interfacing with the members of the National Electric Energy Testing Research and Applications Center to develop and conduct research projects dealing with transmission and distribution issues. For 22 years, he was with Georgia Power Company, engaged in transmission/ distribution system design, construction, operation, maintenance, and automation. Mr. Lambert participates in the IEEE Power and Energy Society Distribution Subcommittee and the PES Switchgear Committee and is serving on the PES Governing Board as the Vice President for Chapters. Deepak Divan (S’78–M’78–SM’91–F’98) received the B.Tech. degree from the Indian Institute of Technology, Kanpur, India, in 1975 and the M.Sc. and Ph.D. degrees from the University of Calgary, Calgary, AB, Canada, in 1979 and 1983, respectively. He is the President and CTO of Varentec Inc., San Jose, CA, USA, which is a company funded by the green-tech venture capital firm Khosla Ventures, providing innovative solutions to achieve a smart and dynamically controllable grid. He has over 35 years of experience in industry and academia in the areas of power electronics applied to utility and industrial systems, with Varentec being his third entrepreneurial venture. From 2004 to 2011, he was a Professor of electrical and computer engineering and the Founding Director of the Intelligent Power Infrastructure Consortium with the Georgia Institute of Technology, Atlanta, GA, USA. Prior to that, he led Soft Switching Technologies as Founder and CEO. He was also a Professor of electrical engineering with the University of Wisconsin–Madison, Madison, WI, USA. He has over 250 papers and 50 issued and pending patents. He combines unique perspectives on the changing landscape of the Transmission and Distribution grid and the need for a transition to dynamic grid control, including advanced power electronics solutions. His research interests include dynamic grid control, sustainable energy, and advanced power electronics. Dr. Divan was a recipient of the 2006 IEEE Newell Award and is a past President of the IEEE Power Electronics Society.