IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 24, NO. 3, SEPTEMBER 2009 673 Experimental Evaluation of Active Filtering in a Single-Phase High-Frequency AC Microgrid Sudipta Chakraborty, Member, IEEE, and Marcelo G. Simões, Senior Member, IEEE Abstract—Population growth in the world along with rapid technological expansion of the society demand efficient, economically viable, and environment-friendly energy conversion systems. The previous theoretical and simulation works have demonstrated that a 500-Hz single-phase high-frequency ac (HFAC) microgrid is a novel step toward integrating renewable energy sources in a distributed generation system. This paper goes one step further in describing the practical implementation of HFAC microgrid with active filters for a small 1-kW system. The protection issues for both the source and series converters are also addressed in this paper by developing a new but simple protection scheme. In the experimental microgrid system, a universal active power line conditioner (UPLC) and a unified power quality conditioner (UPQC) are incorporated to control the power flow and power quality, respectively. Controllers for both the UPQC and UPLC are developed based on the instantaneous single-phase p-q theory, and controlled pulsewidth modulated inverters are then implemented to synthesize the desired compensating waveforms. The experimental results obtained confirm that the HFAC microgrid is a practical and useful step toward successfully integrating distributed renewable energy sources ensuring the improved system utilization. Index Terms—Distributed generation (DG), high-frequency ac (HFAC), microgrid, unified power quality conditioner (UPQC), universal active power line conditioner (UPLC). I. INTRODUCTION HE BACKBONE of any microgrid scheme is the power converters and dedicated controllers in order to have a common output for all the power sources and storage devices and to supply power with adequate quality to the local loads . Additionally, the connection of local distributed generations (DGs) to the utility necessitates stringent power quality and protection requirements in forms of guidelines provided by IEEE 1547 . To accomplish the integration issues, different power system topologies can be used ranging from the primitive series connection, through common dc distribution and 50/60 Hz ac distribution, toward more advanced high-frequency ac (HFAC) distribution. Each one of these topologies has its own merits and demerits, which will be discussed briefly in the paper. It has been shown in the previous works that a single-phase HFAC link, having a common bus that operates at 500 Hz, is a feasible way to integrate renewable sources to the load and the grid . The HFAC-based power electronics system showed its promise for efficient utilization of the microgrid. T Manuscript received April 22, 2008; revised September 29, 2008. First published June 10, 2009; current version published August 21, 2009. This work was supported by the National Science Foundation (NSF). Paper no. TEC-00143-2008. S. Chakraborty is with the National Renewable Energy Laboratory, Golden, CO 80401 USA (e-mail: [email protected]). M. G. Simões is with Colorado School of Mines, Golden, CO 80401 USA (e-mail: [email protected]). Digital Object Identifier 10.1109/TEC.2009.2015998 But on the downside, higher frequency attributes to the higher power losses and voltage drops in the distribution system. The detailed analysis for choosing 500 Hz is done in  where it has been shown that the frequency ranging from 400 Hz to 1 kHz is suitable for several residential, industrial, and commercial applications. For optimum HFAC bus utilization, it is important to compensate reactive power, load current harmonics resulting from nonlinear loads, and voltage distortions resulting from the source and/or converter nonlinearity. A unified power quality conditioner (UPQC), having shunt and series active filters, is a power electronic solution that can accomplish all these functions , . Another very important task in the microgrid is to control the power flow. A universal active power line conditioner (UPLC) also having the series and shunt active filters is a power electronic way to control the power flow between the microgrid and the utility grid , . Both the UPQC and UPLC utilize the instantaneous power theory (also known as p-q theory) to calculate the compensating quantities that are then synthesized by using voltage-source pulsewidth modulated (PWM) inverters. The objective of this paper is to revisit the theoretical aspects of single-phase HFAC microgrid with active filters to corroborate the previous simulation findings with new experimental results. A 1-kW laboratory prototype for the single-phase HFAC microgrid is built for the experimental evaluation. The issues related to the practical implementation of HFAC microgrid such as inverter controls and protections are also discussed in this paper. To demonstrate the operation of such system, the paper presents some interesting experimental results of a single-phase HFAC microgrid, with nominal frequency of 500 Hz. In conclusion, an advanced renewable-generation-based distributed power generation system is developed and experimentally verified under this paper in form of a single-phase HFAC microgrid that provides dependable energy distribution, satisfies power quality demands, and is compatible with residential and small business (RSB) level needs. II. POWER ELECTRONIC INTERFACES FOR MICROGRID The converters required to interface distributed resources into the microgrid can be of different configurations depending on the renewable sources to be integrated. The sources such as photovoltaic (PV) or fuel cell generate dc voltage as the output. On the other hand, wind and microturbines generate ac output often at variable frequencies. Based on the configuration, power electronics interface is required to integrate these outputs into microgrid. Different aspects should be evaluated before selecting the best option for a specific scheme, such as the type of power sources considered, the distance between the 0885-8969/$26.00 © 2009 IEEE Authorized licensed use limited to: COLORADO SCHOOL OF MINES. Downloaded on April 08,2010 at 21:49:01 UTC from IEEE Xplore. Restrictions apply. 674 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 24, NO. 3, SEPTEMBER 2009 Fig. 1. Integration of distributed sources by dc or 60 Hz ac bus. sources and the loads, the requirement for connection with the utility grid, desired level of reliability and power involved, and additional objectives for advanced functions, such as active filtering, power dispatch coordination, or uninterruptible power supply (UPS) functions . The simplest and common type of electrical energy integration is through a dc link where a common dc bus is used for connecting DG sources and storages through appropriate converters, as shown in Fig. 1. The output of each converter, controlled to generate the same level of voltage, is connected in parallel, generating a dc integration bus. The common dc bus is then utilized to convert dc power into single-phase or threephase ac for consumer use. Instead of using dc bus, 60 Hz ac bus is a possibility for the integration of distributed sources. The same structure, as shown in Fig. 1, can be used with the proper choice of converters to convert DG source outputs into 60 Hz ac. This bus, which can be either single-phase or three-phase, can be the utility grid or other independent bus to supply the local loads. In the last case, another converter may be needed to connect the independent bus to the utility grid. A newer integration technology using an HFAC link is also a feasible option for the renewable source integration. The HFAC power distribution system had already been implemented for aerospace applications, and the National Aeronautics and Space Administration (NASA) evaluated this scheme for space station applications. It had been proposed as a viable distribution system for hybrid electric vehicle power distribution as well . In the previous works, the advantages of the HFAC microgrid system are discussed in details along with its possible residential and commercial applications . It is obvious that biggest advantage of the HFAC system lies in its smaller size, higher power density, and modularity. But the choice of frequency is very important for the microgrid as the high frequency causes higher power losses and voltage drops along the line, thus restricting the physical length of the system . Additionally, harmonics are present in the HFAC system due to power converters, nonlinear loads. Therefore, for optimum HFAC bus utilization, it is extremely important to compensate reactive power, load current harmonics, and voltage distortions. Also important is to control power flow between the microgrid and the main grid. Advanced active filtering is capable of achieving all these objectives. A summary of merits and demerits of the different interfaces for the distributed energy applications is given in Table I. As discussed in , the integration of the HFAC microgrid with the 60 Hz utility grid can be achieved by frequency link converters such as PWM-controlled dc-link back-to-back converter or matrix converter without a dc link. The main characteristics for these converters are the bidirectional power flow capability that is required for the proposed system. The 60 Hz loads can be connected to the load bus using the similar converters. Another effective way for connecting 60 Hz loads to the microgrid is to use naturally commutated cycloconverters . Also, the source converters are essential for integrating renewable and distributed sources in the HFAC microgrid. The HFAC microgrid system topology is apparently complicated due to presence of all these converters, but irrespective of the integration topology used (such as dc link or 60 Hz ac link), power electronics converters are always required to integrate distributed sources in the microgrid. As an example, for dclink microgrid, ac–dc or dc–dc converters are used with sources and dc–ac inverters are required for utility/load connection, as shown in Fig. 1. Therefore, loss of efficiencies due to multiple converters is an issue with any microgrid. For HFAC microgrid, series resonant converters, utilizing zero-voltage or zero-current switching, can be used with each of the sources to eliminate switching loss, thus improving converter efficiency . Highfrequency transformer is an additional component in the HFAC system, but it provides galvanic isolation and boosts the HFAClink voltage that permits higher converter and machine terminal voltage ratings for the same output power. The higher level of voltage makes the machine and converter design more efficient. Also important is to note that the p-q-theory-based power flow control and power quality control can be incorporated into the individual interfacing converters, but that will, in turn, cause more complexities in control design for multiple source systems. It may therefore be convenient to lump up the controllers into the form of UPQC and UPLC. Obviously, for the smaller microgrid system, careful economic and implementation considerations should be made before deciding on a particular topology and converters. III. HFAC MICROGRID WITH ACTIVE FILTERS As discussed in the previous section, HFAC systems are already in use for several applications such as aerospace and spacecraft power. Also, frequencies ranging from 400 Hz to 20 kHz were being experimented for power distribution systems . But the use of HFAC systems for the microgrid is a novel concept and requires several considerations to be addressed. The selection of the operating frequency is the first important task to be achieved. The conceptual design of the HFAC microgrid is the next task to be accomplished. Finally, the active filtering algorithms are to be applied for the singlephase HFAC system to control the power flow and the power quality for the microgrid. Authorized licensed use limited to: COLORADO SCHOOL OF MINES. Downloaded on April 08,2010 at 21:49:01 UTC from IEEE Xplore. Restrictions apply. CHAKRABORTY AND SIMÕES: EXPERIMENTAL EVALUATION OF ACTIVE FILTERING IN A SINGLE-PHASE HFAC MICROGRID 675 TABLE I COMPARISON BETWEEN DIFFERENT POWER ELECTRONICS INTERFACES A. Selection of Frequency Two significant factors that affect the suitability of power transmission and distribution at high frequencies are the increased power losses and voltage drop incurred along the line. An analysis using simple single-core coaxial cable with a grounded sheath was used in  for demonstration purposes. It is possible to extend that analysis to any type of cable, provided its geometry and frequency-dependent material properties are known. It has been shown in  that the dominant cause of power loss at frequencies below 10 kHz is the skin effect. The frequency-dependent losses remain constant up to 10 kHz, but after that frequency, the losses increase exponentially . It has also been shown in  that for frequencies below 500 Hz, the voltage drop is mainly due to the series resistance. Above this frequency, the series impedance, represented by the reactance, dominates the voltage drop. Due to these physical limitations of going toward very high frequency, it had been shown that the frequency in the range of 400 Hz–1 KHz is suitable for several residential, industrial, and commercial applications . For the proposed microgrid, a single-core coaxial cable is considered for which 500 Hz is the optimum frequency at which both the resistance and reactance values match making voltage drop minimum . Instead of 500 Hz, common 400 Hz can also be used without any significant difference. The 500 Hz operating frequency is also useful because: 1) as 400 Hz is common for military and space applications, power converters can be found in the market that can be used in a 500-Hz system with very small modifications; 2) at this frequency, power losses and voltage drop are not very drastic, allowing a longer bus compared to higher 10–20 kHz systems; 3) reductions in size and volume are still attractive; and 4) implementation of active filters is feasible with the current power electronic devices and controllers. B. Single-Phase HFAC Microgrid In Fig. 2, the proposed single-phase HFAC-based microgrid is shown along with the UPQC and UPLC. There are four highfrequency buses in the microgrid system, as described next. In the source bus (or Bus 1), a variety of renewable sources are connected along with energy storage systems. The utility grid is connected in utility connection bus (or Bus 2) through a bidirectional converter and associated loads. In load bus (or Bus 3), high-frequency loads are connected. The intermediate supply bus gets its supplies from Bus 1 and Bus 2, and then sends the power to Bus 3 (load bus). Two static transfer switches (STSs) are present in the proposed microgrid structure to provide islanded mode of operation of the microgrid in case of grid failure. The integration of the single-phase HFAC microgrid with the three-phase 60 Hz utility grid and with the consumer loads is discussed in . In this paper, the high-frequency loads are only considered, which represent the loads that can be connected directly to the 500 Hz HFAC microgrid based on the applications discussed in . Bus 1 and Bus 2 are connected by a controlled distribution line through a UPLC. The main function of the UPLC is to control the power flow between the source bus and the utility connection bus. The UPLC also mitigates current harmonics present in the utility connection bus due to the connection of the bidirectional utility converter. To maintain the power balance of the whole system with UPLC, another uncontrolled distribution line is required. In the proposed microgrid structure, the intermediate supply bus, connected to both Bus 1 and Bus 2, works as the uncontrolled line. The voltage at the intermediate supply bus is distorted as the integration of all sources adds source voltage harmonics. Also, the loads connected across the load bus cause high level of harmonic content in the current coming out of the intermediate supply bus. The UPQC integrated in the uncontrolled distribution line imposes that the voltage at the load bus is harmonic-free. It also compensates load current harmonics and reactive power in a way that the total current coming out of the intermediate supply bus is also harmonic-free and in phase with the fundamental source voltage, resulting in unity power factor. C. Advanced Active Filtering The instantaneous power theory (or p-q theory) was developed by Akagi et al.  for the three-phase system where Authorized licensed use limited to: COLORADO SCHOOL OF MINES. Downloaded on April 08,2010 at 21:49:01 UTC from IEEE Xplore. Restrictions apply. 676 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 24, NO. 3, SEPTEMBER 2009 Fig. 2. Single-phase HFAC microgrid with UPLC and UPQC. detecting harmonic current requires instantaneous values of three voltages and three currents. But for the single phase, only one voltage and current exist, thus making the p-q calculation different. From the computational viewpoint, for a balanced three-phase system knowing voltage and current for one phase can lead to finding voltages and currents for the other two phases by using the delays of 2π/3 and 4π/3, respectively. Liu et al. utilized this approach for applying p-q theory in a single-phase system . In their method, it is necessary to obtain an instantaneous π/2 phase lag of the current and voltage waveforms to define a pseudo two-phase system . The instantaneous power theory can then be applied to design active filters for the single-phase system. The method described by Liu et al. is based on the calculation of the fundamental components in the α–β coordinates, which needs an intermediary calculation. However, as presented by Watanabe et al.  for three-phase system, the compensating current and voltages in α–β coordinates can be obtained directly, based on the power components to be compensated. This approach is adapted for development of active filters in the proposed single-phase HFAC microgrid system. 1) Universal Active Power Line Conditioner: The UPLC in the HFAC microgrid is used for controlling the active and reactive power flow (by series converter) along with the mitigation of current harmonics that are generated due to utility connection (by shunt converter). First, a phase-locked loop (PLL) determines the angular fundamental frequency ω of the HFAC system. It is then utilized to obtain the fundamental components Vα , Vβ corresponding to the voltage V in the α–β system . For the shunt active filter in UPLC, the utility load current components are obtained in a way such that iL α corresponds to iL and iL β corresponds to iL delayed by 90◦ . Then, the instantaneous active and reactive power can be calculated as p Vα iL α p̄ p̃ Vβ = = + . (1) q q̄ q̃ −Vβ Vα iL β The compensating reference currents in α–β coordinates can be defined by the inversion of (1) , . In (2), the compensation of the current harmonics and instantaneous reactive power is considered ∗ 1 Vα −Vβ p̃ icα = 2 . (2) 2 q̃ i∗cβ V V Vα + Vβ α β After calculating the compensating reference current in α–β coordinates, the resulting compensating current must be converted back to a–b–c coordinates. In a single-phase system, this current is defined only by the α-component, which is the one in phase with the utility load current. In this way, the reference current for the UPLC is calculated by i∗c = i∗cα . (3) The calculated current (i∗c ) is then used as the reference for a hysteresis-current-controlled voltage-source PWM inverter, which supplies the actual compensating current ic . The series active filter in UPLC controls the power flow along with the voltage harmonics mitigation. In the controller, the inputs are the source current is and the fundamental voltage com , Vsβ . The calculations of these fundaponents Vα , Vβ and Vsα mental voltages are shown in , . The source current is is Authorized licensed use limited to: COLORADO SCHOOL OF MINES. Downloaded on April 08,2010 at 21:49:01 UTC from IEEE Xplore. Restrictions apply. CHAKRABORTY AND SIMÕES: EXPERIMENTAL EVALUATION OF ACTIVE FILTERING IN A SINGLE-PHASE HFAC MICROGRID time-delayed to get the β-component whereas the α-component is the source current itself. In a two-generator system, the active power flow can be controlled by inserting voltage in series that leads source voltage by 90◦ . When the inserted voltage is in phase with the source voltage, reactive power flow can be controlled , . This idea of power flow control is utilized in the series active filter of UPLC. The instantaneous harmonic power calculation is same as in case of the shunt converter ph Vα isα Vβ = . (4) qh −Vβ Vα isβ The high-pass filters of cutoff frequency 500 Hz are used to get the oscillating components of the instantaneous power. These oscillating powers p̃h and q̃h correspond to the harmonics present in the source voltage. To incorporate the idea of power flow control, the calculated ph and qh from the system are compared with the reference pref and qref . The errors are then fed to the proportional–integral (PI) controllers to generate two signals q̄c and p̄c , respectively. The generated q̄c inserts a fundamental voltage component orthogonal to the voltage V , thus controlling the active power flow. Similarly, p̄c inserts a compensating voltage in phase with the voltage V to control the reactive power flow. The reference compensating harmonic current for the series active filter can now be directly calculated as ∗ 1 Vα −Vβ p̃h + p̄c ihα = . (5) 2 i∗hβ Vα q̃h + q̄c Vα2 + Vβ Vβ After calculating the reference compensating harmonic current in α–β coordinates, it must be converted back to a–b–c coordinates. In single-phase system, this current is defined only by the α-component, which is the one in phase with the source current. In this way, the reference compensating harmonic current for the UPLC is obtained by i∗h = i∗hα (6) and the compensating harmonic voltage is calculated as Vh∗ = ki∗h (7) where k is a fictitious resistance included in the series active filter calculation. Finally, the reference voltage for the UPLC series filter is calculated as . Vc∗ = Vs + Vh∗ − Vsα (8) The voltage-controlled voltage-source inverter then uses the voltage reference Vc∗ to supply the actual compensating voltage Vc . 2) Unified Power Quality Conditioner: The UPQC is integrated in the system to make the load bus voltage harmonic-free. It also acts on the total load current, to compensate for current harmonics and reactive power, in a way that the total current coming out of the intermediate supply bus is harmonic-free and in phase with the fundamental source voltage, resulting in a unity power factor. The shunt active filter in the UPQC works with similar principle as that of the UPLC. The inputs to the shunt controller are 677 the intermediate bus voltage Vhf and the load current iL oad1. By using similar calculations, as in (1)–(3), it then generates the reference current i∗c1 . This reference current is then fed to a hysteresis-current-controlled voltage-source inverter to generate actual compensating current ic1 . For the series active filter in UPQC, a PLL is used to obtain the fundamental components corresponding to the current ihf in the α–β system. The α–β components of source voltage Vhf are obtained in the way such that Vhf ,α corresponds to Vhf and Vhf ,β corresponds to Vhf delayed by 90◦ . Then, first the power components are calculated using (1). The fundamental voltage components in the α–β coordinates are calculated as ∗ 1 Vf α ihf ,α p̄hf ihf ,β = 2 . (9) Vf∗β q̄hf ihf ,α + i2hf ,β ihf ,β −ihf ,α The fundamental voltage in a–b–c coordinates is defined by the α-component only, which is the one in phase with the supply voltage. The compensating reference voltage is then defined as ∗ = Vhf − Vf∗ = Vhf − Vf∗α . Vc1 (10) ∗ The series PWM inverter for UPQC then uses the reference Vc1 to supply the actual compensating voltage Vc1 . IV. EXPERIMENTAL SETUP An experimental prototype is built to verify the operation of a single-phase HFAC microgrid with UPQC and UPLC. The control circuit for the HFAC microgrid is developed in a PC equipped with data acquisition board (DAQ) and digital I/O (DIO) board. The Disk Operating System (DOS) program environment is chosen because it is simpler for real-time applications with user-defined interrupts. For this paper, DAQ board PCI 4520 from RTD Embedded Technologies, Inc., is used . A hardware PLL circuit is developed that uses MAX038 to generate an inverted cosine waveform, which, in fact, is the voltage Vβ . This output signal has a 90◦ phase shift to the fundamental of the input signal. The calculations based on p-q theory generate the reference compensating currents and reference compensating voltages. Based on these reference values and actual currents and voltages from the converters, controllers are implemented to get the duty cycle output from the PC. The generated duty cycle values are then sent to the PIC microcontroller, PIC 18F452 from Microchip , to generate PWM for the converters. For fast transmission of the duty cycle data from computer to PIC, DIO board DIO96H is used . The power circuit part consists of six single-phase H-bridge converters, two for the source, and four for the series and shunt converters of the UPQC and UPLC. For the source converter, a dc supply is used to emulate renewable sources such as PV or fuel cell. The PIC microcontrollers for the sources use lookup table for duty cycles and generate PWM based on these duty cycles so that a 500-Hz ac voltage is obtained at the output of the source converter. For both the UPQC and UPLC, the shunt and series active filter converters are connected back-to-back, with a common dc link. In the dc bus, a capacitor is connected and the voltage control for this capacitor is incorporated in the Authorized licensed use limited to: COLORADO SCHOOL OF MINES. Downloaded on April 08,2010 at 21:49:01 UTC from IEEE Xplore. Restrictions apply. 678 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 24, NO. 3, SEPTEMBER 2009 Fig. 3. Overvoltage protection for the source converter. (a) Circuit diagram. (b) Placement in the distribution line. Fig. 4. Block diagram for overcurrent shutdown circuit. shunt active filter calculations. High-frequency transformers are used to inject voltages from the series converters into the HFAC microgrid. A protection scheme is required for the series converter since it cannot be protected with normal circuit breakers or power fuses. The previous papers  mainly concentrated on the protection of the series converter itself during the short circuit conditions of the power distribution system that causes larger currents to flow through the primary of the series transformer (distribution side), generating high voltages and currents in the secondary windings (series inverter side) and damaging the series converter. But for the microgrid system, where the series converter is connected between the source and the loads, it is also important to protect the source converters from the possible malfunctioning of the control system that can cause high voltage at the secondary of the series transformer. In the experimental prototype, a protection scheme consisting of two simple protection circuits is developed. A. Overvoltage Protection for Source Converter The circuit shown in Fig. 3 operates on a very simple principle. When any malfunction in the series converter causes high voltage across the transformer winding that is greater than the positive threshold or less than the negative threshold, as set by the zener diodes, the respective thyristors get triggered causing high instantaneous current to blow the fuse and disconnect the source from supplying to the load. used to trigger the MOSFET switches. According to the design of the power boards, the low logic level signals from the control circuits enable the gate drivers and, in turn, enable the converters . A debouncing circuit is used to obtain a digital electronic key to turn on and off the series converter. The output of this electronic key is sent to a digital input of the PIC microcontroller. The PIC program generates the PWM as well as uses the state of this input to enable or to disable the converter of the series active filter. The logic signal low from this circuit denotes the enabling signal, as shown in the “From ON/OFF debouncing switch” signal in Fig. 4. In the normal state of operation, this is the signal that controls the ON/OFF of the series converter. But when the series converter current increases over the threshold value (shown by “Ref” in Fig. 4), the logic circuit as shown in the figure generates a high logic signal that shuts down the series converter. Once shut down, the only way to start it again is to manually press the “RESET” switch, as shown in Fig. 4. This protection circuit is useful for the abnormal conditions in the line, such as line faults, that result in a large current from the series converter. One important advantage of the circuit shown here is that the threshold values for the positive and negative peak converter currents can be set individually, and thus can be used for unsymmetrical protection for positive and negative cycles . The annotated picture of the complete experimental setup is shown in Fig. 5 denoting each of the important parts of the system. V. EXPERIMENTAL RESULTS A 1-kW laboratory prototype for the single-phase HFAC microgrid is built for the experimental evaluation. The main goal for this study is to validate the operation of the HFAC microgrid along with the advanced active filtering. A. Test System The test system is developed similar to the one shown in Fig. 2. The system voltage is set at 30 V, 500 Hz. In the source bus of HFAC microgrid, a dc–ac converter is connected that generates 500 Hz voltage along with 15% third harmonics. In the utility connection bus, another source converter is connected generating 500 Hz voltage without any harmonics. In the utility connection bus, a diode bridge is connected through a 0.5-mH inductance. The output of the diode bridge is then connected to a parallel combination of 10 Ω resistor and 1 µF capacitor. This 500 Hz source, diode bridge and loads in the utility connection bus are used to emulate the grid-connected converter. In the load bus, nonlinear loads are connected consisting of a series connection of a diode, 0.2 mH inductor and 10 Ω resistor. As mentioned in the previous section, the p-q-theory-based active filtering equations (1)–(10) are implemented in PC equipped with DAQ and DIO cards. B. Overcurrent Shutdown for Series Converter The overcurrent protection circuit, as shown in Fig. 4, utilizes the enabling/disabling of the gate driver. For the laboratory setup, the gate drivers IR2104 from International Rectifier are B. HFAC-Link Generation For generation of the HFAC link, a dc–ac converter and a PIC microcontroller are used. In the PIC microcontroller, a table of Authorized licensed use limited to: COLORADO SCHOOL OF MINES. Downloaded on April 08,2010 at 21:49:01 UTC from IEEE Xplore. Restrictions apply. CHAKRABORTY AND SIMÕES: EXPERIMENTAL EVALUATION OF ACTIVE FILTERING IN A SINGLE-PHASE HFAC MICROGRID Fig. 5. 679 Picture of the complete experimental setup. Fig. 7. Screenshot of the voltages taken from oscilloscope. C. UPQC Series Active Filter Fig. 6. HFAC source bus voltage. (a) Waveform. (b) Frequency spectrum. duty cycles is loaded and the duty cycle values are sent to the source converter driver at a particular sampling rate defined by the interrupts in the microcontroller to generate 500 Hz sine wave. The amplitude of the voltage depends on the dc input voltage. The voltage output from the source converter at source bus is given in Fig. 6(a). The frequency spectrum of the voltage waveform shows the presence of third harmonics in it, as given in Fig. 6(b). The amplitude of the third harmonics is 4.2 V, which is approximately 15% of the amplitude of the fundamental (which is around 30 V). This is expected, as for the source converter, the duty cycles are designed in such a way that the output voltage should be polluted with 15% third harmonics. In Fig. 7, the first channel of the oscilloscope shows the voltage at the intermediate supply bus before compensation. The load bus voltage is shown in the second channel after compensation. These waveforms are taken after voltage sensors with a gain of (1/50). It can be seen from the screenshot that most of the third harmonics is eliminated from the source voltage by the UPQC series active filter, so that the voltage at load bus is approximately sinusoidal. D. UPQC Shunt Active Filter In the screenshot given in Fig. 8, the load bus current is shown in the first channel and the intermediate supply bus current is in the second channel. The currents are taken after current sensors that have dc offsets of 5 V and gains of 0.625. The frequency spectrums of the currents are shown in Fig. 9, where top curve is for load current (Iload) and the bottom curve is for intermediate supply bus current (Ihf). It is apparent from the figures that the Authorized licensed use limited to: COLORADO SCHOOL OF MINES. Downloaded on April 08,2010 at 21:49:01 UTC from IEEE Xplore. Restrictions apply. 680 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 24, NO. 3, SEPTEMBER 2009 Fig. 8. Screenshot of the currents taken from oscilloscope. Fig. 11. Screenshot of power flow in controlled distribution line. (a) Active power. (b) Reactive power. Fig. 9. Frequency spectrums of load current and intermediate bus current. voltage in the controlled distribution line. For this experiment, the reactive power reference is made constant at −250 VAR whereas the active power reference is changed in step from a small value of 30 W to a large value of 300 W. In Fig. 11, the calculated active and reactive power waveforms are shown. From the figure, it can be observed that the power flows through the controlled distribution line are following the reference values within a small steady-state error. F. UPLC Shunt Active Filter Fig. 10. Phase relations between voltage and currents. shunt active filter eliminates the dc offset and mitigates most of the current harmonics. The shunt active filter also compensates the reactive power. Though the load current (Iload) is lagging, the current coming out of the intermediate supply bus (Ihf) is in phase with the intermediate bus voltage (Vhf), as shown in Fig. 10. E. UPLC Series Active Filter The UPLC series active filter in the HFAC microgrid controls the active and reactive power flow from the source bus to the utility connection bus through injection of a series compensating In the actual microgrid, utility connection bus will be used to connect microgrid with the utility through frequency changing bidirectional converter such as matrix converter. Therefore, inevitably, the current coming from this bus will have current harmonics. For the laboratory experiment, the utility grid and inverter are represented by the parallel association of a clean HFAC voltage source and loads constituting a diode bridge, an inductance, a resistance, and a capacitance. The harmonics present in the utility bus load current (IL ) are mitigated by the UPLC shunt active filter so that the source current is cleaner. In Fig. 12, the screenshots for the currents are given where the first channel shows the current from source bus converter and the second channel shows the utility load current. A random load disturbance is created for short duration (for about five cycles) by using a parallel resistive load controlled by an electronic relay. The currents waveforms are taken after current sensors that have dc offsets of 5 V and gains of 0.625. It can be observed from Fig. 12 that the source current has considerably less dc offset and harmonic pollution in it compared to the utility load current. Considering that the source is supplying both the nonlinear loads present in load bus and Authorized licensed use limited to: COLORADO SCHOOL OF MINES. Downloaded on April 08,2010 at 21:49:01 UTC from IEEE Xplore. Restrictions apply. CHAKRABORTY AND SIMÕES: EXPERIMENTAL EVALUATION OF ACTIVE FILTERING IN A SINGLE-PHASE HFAC MICROGRID 681 controllers; there are noises in the sensor readings; and finally, the high-frequency transformers are different than the simulation. Still, from the experimental results, most of the concepts associated with the HFAC microgrid and the active filtering are validated. A DOS-based controller is used due to its availability in the laboratory, but it contributes the noise and inaccuracy in the experimental results. A more advanced DSP-based controller is expected to give better results. VI. CONCLUSION Fig. 12. Oscilloscope screenshot of source current and utility load current. Fig. 13. Phase relations between source bus voltage and current. utility connection bus, such a small amount of harmonics in the source bus current clearly indicates that the shunt active filters both in UPLC and UPQC successfully mitigate the higher order harmonics from the source currents. Another interesting fact is that, though the reactive power flow through the controlled distribution line can be controlled, the reactive power has no effect in the source current, since it is generated solely by the UPLC series converter. As the shunt UPQC compensates the reactive power due to loads in the load bus and shunt UPLC compensates the reactive power due to utility loads in the utility connection bus, no reactive power generation from the source is required. Therefore, the source current (Isource1) is in phase with the source voltage (Vsource1), as shown in Fig. 13. The simulation results in  have already shown that the single-phase HFAC microgrid is as an exciting step toward integration of renewable energy sources in a DG system. But in simulation, many simplifying assumptions are typically made such as ideal power electronic devices, lossless distribution lines, and no sensor noises. Therefore, it is important to build a laboratory prototype to observe the real-world implementation of the proposed system. Though the experimental system is built as close as possible to the simulated system, several additional factors are present in the experimental setup. The PWM generation is different in the experimental setup; there are significant line inductances present; the sampling frequency used in the experiment is only 10 kHz due to choice of particular DAQ board, which, in turn, affects the p-q calculations and the PI An HFAC microgrid is a feasible way for integrating renewable energy sources when the active filters are incorporated to control the power quality and power flow. The objective of this paper is to revisit the theoretical aspects of single-phase HFAC microgrid with active filters to support the previously published simulation findings with new experimental results. The successful implementation of HFAC microgrid with adequate power flow and power quality control based on the single-phase p-q theory ensures best utilization of the HFAC system. Both for UPLC and UPQC, PWM inverters are implemented to synthesize the desired compensating waveforms based on the reference voltage and currents coming from series and shunt controllers, respectively. The experimental results show that the power quality problems are mitigated by the shunt and series active filters present in form of UPQC. It is also evident from the results that the current from the intermediate supply bus becomes approximately sinusoidal and in phase with the intermediate bus voltage. The voltage distortions present at that bus are compensated, so that the load voltage is almost harmonic-free. Besides, the experimental results have shown that using the series active filter in UPLC, the active and reactive power flow are controlled between the renewable sources and utility grid. Alongside, the shunt active filter in the UPLC effectively compensates the utility load current harmonics and reactive power. Though better results are possible using the DSP-based controller, by this paper, the HFAC microgrid concept is demonstrated to be useful toward successfully integrating and utilizing the distributed renewable energy sources. REFERENCES  F. A. Farret and M. G. Simões, Integration of Alternative Sources of Energy. Piscataway, NJ: Wiley/IEEE Press, 2006.  B. Kroposki, C. Pink, R. DeBlasio, H. Thomas, M. Simoes, and P. K. Sen, “Benefits of power electronic interfaces for distributed energy systems,” in Proc. IEEE Power Eng. Soc. Gen. Meeting, Montreal, QC, Canada, Jun. 2006, pp. 1–8.  S. Chakraborty and M. G. Simões, “Advanced active filtering in a single phase high frequency AC microgrid,” in Proc. IEEE Power Electron. Spec. Conf. (PESC 2005), Recife, Brazil, Jun.12–16, pp. 191–197.  S. Chakraborty, M. D. Weiss, and M. G. Simoes, “Distributed intelligent energy management system for a single phase high frequency AC microgrid,” IEEE Trans. Ind. Electron., vol. 54, no. 1, pp. 97–109, Feb. 2007.  H. Fujita and H. Akagi, “The unified power quality conditioner: The integration of series and shunt- active filters,” IEEE Trans. Power Electron., vol. 13, no. 2, pp. 315–322, Mar. 1998.  A. Aredes, K. Heumann, and E. H. Watanabe, “An universal active power line conditioner,” IEEE Trans. Power Del., vol. 12, no. 2, pp. 545–551, Apr. 1998. Authorized licensed use limited to: COLORADO SCHOOL OF MINES. Downloaded on April 08,2010 at 21:49:01 UTC from IEEE Xplore. Restrictions apply. 682 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 24, NO. 3, SEPTEMBER 2009  B. K. Bose, M.-H. Kim, and M. D. Kankam, “High frequency ac vs. dc distribution system for next generation hybrid electric vehicle,” in Proc. IEEE-IECON, 1996, vol. 2, pp. 706–712.  H. Akagi, Y. Kanazawa, and A. Nabae, “Generalized theory of the instantaneous reactive power in three-phase circuits,” in Proc. Int. Power Electron. Conf. (IPEC 1983), Tokyo, Japan, pp. 1375–1386.  J. Liu, J. Yang, and Z. Wang, “A new approach for single-phase harmonic current detecting and its application in a hybrid active power filter,” in Proc. IEEE-IECON, 1999, vol. 2, pp. 849–854.  E. H. Watanabe, R. M. Stephan, and M. Aredes, “New concepts of instantaneous active and reactive powers in electrical systems with generic loads,” IEEE Trans. Power Del., vol. 8, no. 2, pp. 697–703, Apr. 1993.  S. Chakraborty, “Control and intelligent optimization of distributed renewable energy sources integrated in a single phase high frequency AC microgrid,” Ph.D. dissertation, Colorado School of Mines, Golden, 2007.  PCI 4520 Driver Manual, version 5.02, PCI4520/DM7520 Data Acquisition Driver, Real Time Devices USA, Inc., State College, PA, 2003.  High Performance, Enhanced Flash Microcontrollers With 10-bit A/D, PIC 18FXX2 Datasheet, Microchip Technology, Inc., Mountain View, CA, 2001.  High-Density, Logic-Level Digital I/O Board, PCI-DIO96H User’s Guide, rev. 2.0, Measurement Computing, Norton, MA, 2006.  L. A. Moran, I. Pastorini, J. Dixon, and R. Wallace, “A fault protection scheme for series active power filters,” IEEE Trans. Power Electron., vol. 14, no. 5, pp. 928–938, Sep. 1999. Sudipta Chakraborty (S’02–M’07) received the B.E. degree in electrical engineering from Bengal Engineering College (now Bengal Engineering and Science University), Shibpur, India, in 2001, and the Ph.D. degree in engineering systems, electrical specialty, from Colorado School of Mines, Golden, in 2007. He joined the National Renewable Energy Laboratory, a national laboratory of the U.S. Department of Energy, Golden, where he is currently engaged in several power-electronics-related projects. His current research interests include integration and testing of DG into utility grid using high power electronics systems, power converter control algorithms, intelligent fuzzy-neural controls, and economic optimization techniques. Dr. Chakraborty was the recipient of several awards and scholarships from the Government of India for his scholastic achievements. He received the Myron Zuker Travel Grant from the IEEE Industrial Applications Society to attend the IEEE Industry Applications Society (IAS) Annual Meeting, Salt Lake City, UT, 2003. He was also awarded with the IEEE Industrial Electronics Society Scholarship in 2005 to attend and present his paper at the Industrial Electronics Society (IECON) 2005, Raleigh, NC. Marcelo G. Simões (S’89–M’95–SM’98) received the B.S. and M.Sc. degrees in electrical engineering from the University of São Paulo, São Paulo, Brazil, in 1985 and 1990, respectively, the Ph.D. degree in electrical engineering from the University of Tennessee, Knoxville, in 1995, and the Livre-Docencia (D.Sc.) degree in mechanical engineering from the University of São Paulo, in 1998. He joined Colorado School of Mines, Golden, where he has been engaged in establishing research and education activities in the development of intelligent control for high power electronics applications in renewable and distributed energy systems. He has authored the books Renewable Energy Systems: Design and Analysis With Induction Generators (CRC Press) and Integration of Alternative Sources of Energy (Wiley/IEEE Press). Dr. Simões is the recipient of the National Science Foundation (NSF) Faculty Early Career Development (CAREER) in 2002, the NSF’s most prestigious award for new faculty members. He is serving IEEE in several capacities and is currently an Associate Editor of the IEEE TRANSACTIONS ON POWER ELECTRONICS and the Chair for the IEEE Industry Applications Society (IAS) Industry Automation Control Committee (IACC). Authorized licensed use limited to: COLORADO SCHOOL OF MINES. Downloaded on April 08,2010 at 21:49:01 UTC from IEEE Xplore. Restrictions apply.