Experimental Evaluation of Active Filtering in a Single-Phase High-Frequency AC Microgrid

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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 [1].
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 [2]. 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 [3]. 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 [4] 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 [3], [5]. 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 [3], [6]. 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
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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 [1].
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 [7].
In the previous works, the advantages of the HFAC microgrid
system are discussed in details along with its possible residential
and commercial applications [4]. 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 [4]. 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 [4], 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 [4]. 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 [3]. 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 [7]. 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.
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CHAKRABORTY AND SIMÕES: EXPERIMENTAL EVALUATION OF ACTIVE FILTERING IN A SINGLE-PHASE HFAC MICROGRID
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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 [4] 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 [4] 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 [4]. It has
also been shown in [4] 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 [4].
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 [4]. 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 [4]. 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 [4].
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. [8] for the three-phase system where
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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 [9]. 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 [9]. 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. [10] 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 [11].
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) [3], [11]. 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 [11].
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 [10], [11]. The source current is is
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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 [3], [5]. 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 [11].
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 [11].
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 [12]. 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 [13], to generate PWM for the converters. For fast
transmission of the duty cycle data from computer to PIC, DIO
board DIO96H is used [14].
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
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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 [15] 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 [11]. 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 [11]. 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
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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
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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
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CHAKRABORTY AND SIMÕES: EXPERIMENTAL EVALUATION OF ACTIVE FILTERING IN A SINGLE-PHASE HFAC MICROGRID
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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 [3] 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.
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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).
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