This paper proposes active frequency drift (AFD) as an antiislanding method applied to microinverters with uncontrollable reactive power. When using ordinary inverter topologies, such as full bridge inverters in photovoltaic systems, the islanding phenomenon can be detected with reactive powerbased methods, such as reactive power variation. However, when the inverter topology cannot control the reactive power, conventional antiislanding methods with reactive power cannot be utilized. In this work, the topology used in this paper cannot control the reactive power. Thus, an antiislanding method that can be used in topologies that cannot control the reactive power is proposed. The conventional antiislanding method of the topology that cannot control reactive power is introduced and analyzed. Unlike the conventional AFD method, the proposed method extends a zero current interval every predetermined cycle. The proposed method offers certain advantages over conventional AFD methods, such as total harmonic distortion. The proposed method is validated through simulation and experiment.
I. INTRODUCTION
Fossil fuels and greenhouse gas emissions have been cited as major causes of climate change and environmental pollution. The gradual reduction of fossil fuel use is now a major issue
[1]
. Recently, smallscale power systems have attracted attention. In microgrid systems, electric power generated from distributed generation (DG), such as solar energy or wind power, can be supplied to a local load independently and to a connected main grid in cases of surplus power
[2]
,
[3]
.
In these smallscale power systems, the DC power generated by renewable energy must be converted to AC power prior to grid connection. Accordingly, research into inverters and DC–AC conversion systems has been actively conducted
[4]

[7]
.
In DGs that are directly connected to a grid, the islanding phenomenon can occur during unplanned disconnections from the grid. Specifically, islanding occurs when the gridconnected inverter of the DG and part of the local load are disconnected from the rest of the grid. If this phenomenon is not detected and the generation from a photovoltaic (PV) or other energy source remains operative, the islanded DG system remains energized by the inverter. The islanding phenomenon in PV systems is one of the most important problems in this area because it poses a safety hazard to repair and maintenance personnel. Moreover, islanding may damage inverters and loads in cases of unsynchronized reclosers of the main grid owing to a phase difference in voltage between the grid and the inverter
[8]

[10]
.
Several antiislanding methods have been developed in the past decade. Antiislanding methods resident in inverters have been classified as passive and active, whereas methods nonresident in inverters can be obtained using a specific communication system between the point of common coupling (PCC) and the inverter as shown in
Fig. 1
[11]
,
[12]
. Passive techniques measure voltage, frequency, harmonic characteristics, and so on. Equations (1) and (2) describe the active and reactive power being consumed by the local load shown in
Fig. 1
. Equations (1) and (2) show that if the active or reactive power production of the PV system and the active or reactive power demand of the local load are mismatched in the island, the magnitude or frequency of the voltage at the PCC must change until
P_{load}
=
P
or
Q_{load}
=
Q
[13]
.
PV system overview.
However, when the output power of the DG is equivalent to the local load, a nondetection zone (NDZ) emerges. If the output power falls in the NDZ, passive techniques fail to detect islanding. Such drawback is addressed through active techniques. In most cases, active techniques are more effective and robust than passive ones. Thus, in most gridconnected inverters, active techniques are more frequently used than passive techniques
[14]
,
[15]
.
Most inverters can control their reactive power. Therefore, most inverters detect islanding by applying reactive power variation (RPV). However, inverters that cannot control the reactive power depending on the switching method cannot apply RPV. In this case, other methods must be used.
Conventionally, in inverters that cannot control reactive power, methods that periodically change the active power of the inverters are used. However, in such methods, the inverters can be stopped in the maximum power point tracking (MPPT) control if the active power varies excessively.
This paper proposes a novel active frequency drift (AFD) method with a pulsation of zero current intervals. The applied switching method of a bridgeless inverter that cannot control the reactive power is used to verify the proposed algorithm.
This paper is organized into six sections. Section II presents the operational principle of the topology used in this paper. Section III describes the principles of the proposed method. Sections IV and V present the simulation and experimental results, respectively. Section VI provides the conclusions.
II. OPERATION PRINCIPLE OF THE TOPOLOGY
 A. Basic Operation Principle of SinglePhase Bridgeless Inverter
Fig. 2
shows the bridgeless inverter topology used in this paper. Unlike the conventional bridgeless inverter topology, the topology used in this paper comprises two additional switches. The additional switches are connected in parallel to the diode in the pole as depicted in
Fig. 3
. The proposed topology minimizes the ground leakage current of the panel. This topology can also achieve a relatively high efficiency because the inductors only carry current in the half cycle
[19]
.
Schematic diagram of the bridgeless inverter.
Switching state of the bridgeless inverter.
The switching method of the inverter is operated by a sinusoidal wave obtained from the grid voltage. The sinusoidal wave of the grid voltage is obtained by dividing the grid voltage by its magnitude; it is expressed as
If the current reference is obtained with (5), the output current of the inverter shares the same phase angle as the grid voltage. Therefore, during a normal gridconnected operation, the inverter normally operates with a unity power factor, is synchronized with the grid voltage, and operates at the grid frequency
[8]
.
In the case of a positive half cycle in the sinusoidal wave,
S_{1}
is always turned on, whereas
S_{4}
is repeatedly turned on and off to track the current reference via pulse width modulation (PWM). Similarly, in the case of a negative half cycle in the sinusoidal wave,
S_{2}
is always turned on, whereas
S_{3}
is repeatedly turned on and off to track the current reference via PWM.
Fig. 3
presents the switching states of the bridgeless inverter
[16]
,
[17]
.
When the inverter is controlled with the above method, it cannot control the reactive power, as the topology is controlled with a unity power factor. If the grid voltage is synchronized to active power, the output current of the inverter is also synchronized to active power. Therefore, the inverter topology shown in
Fig. 2
cannot control the reactive power.
 B. Current Path of the Topology Based on Switching States
The current paths of the bridgeless inverter used in this paper are categorized into four types according to switching states. In the positive half cycle of the sinusoidal wave (i.e., when the grid is positive),
S_{1}
and
S_{4}
operate. In
Fig. 4
,
S_{1}
and
S_{4}
are turned on. In this case, the DC power source in the input stage supplies power to the output stage and stores energy in the inductor. In
Fig. 5
,
S_{1}
is turned on, and
S_{4}
is turned off. Freewheeling current flows to the grid using the inductor energy that is stored while
S_{4}
is turned on.
Current path when S_{1}and S_{4} are turned on.
Current path when S_{1}is turned on and S_{4} is turned off.
In the negative half cycle of the sinusoidal wave (i.e., when the grid is negative), current flows in the same way as that in the positive half cycle.
S_{2}
and
S_{3}
operate in the negative half cycle. In
Fig. 6
,
S_{2}
and
S_{3}
are turned on. The DC power source in the input stage supplies power to the output stage and stores energy in the inductor, as in the positive cycle. In
Fig. 7
,
S_{2}
is turned on, and
S_{3}
is turned off. Freewheeling current flows to the grid using the inductor energy that is stored while
S_{3}
is turned on.
Current path when S_{2} and S_{3} are turned on.
Current path when S_{2} is turned on and S_{3} is turned off.
 C. Switching Method Applied to the Topology
The topology shown in
Fig. 2
is controlled by dividing the sinusoidal wave (i.e., the grid voltage) into the positive and negative half cycles. However, if this method is directly applied to the topology, excessive surge current occurs at the zero crossing point of the grid voltage. Such current can cause the breakdown of switches and adversely affect the total harmonic distortion (THD) of the output current. Such surge current must be eliminated.
The modified switching method is applied to the topology to eliminate the surge current. In the modified switching method, the dead time is injected to the switches around the zero crossing point of the grid voltage. All switches are then turned off during the dead time, thus eliminating the surge current.
The sinusoidal wave (i.e., grid voltage) is divided into several sectors to inject dead time as shown in
Fig. 8
. On the basis of the sinusoidal wave,
90°
≤
ωt
≤
270°
is defined as
index 1
, and the remaining interval is defined as
index 2
. The index is divided by phase angle to classify the direction of the sinusoidal wave as increasing or decreasing. As shown in
Fig. 8
, the slope of the sinusoidal wave in
index 1
is negative, whereas that in
index 2
is positive.
Switching method of the topology.
In the latter case,
D_{1}
and
D_{2}
are defined. Unlike in
indexes 1
and
2
, which are classified in terms of the increasing or decreasing direction of the sinusoidal wave,
D_{1}
and
D_{2}
remain constants when determining the amount of dead time. This condition is necessary because switching occurs when
D_{1}
and
D_{2}
intersect with the sinusoidal wave, in which case dead time increases or decreases, that is,
D_{1}
and
D_{2}
increase or decrease, respectively (
Fig. 8
).
If
D_{1}
<
sin(ωt)
≤
D_{2}
in
index 1
and the conventional switching method in subsection A is applied to the topology, the switches operate in the positive half cycle. In the modified switching method, highfrequency switch
S_{4}
is turned off first. When 
D_{1}
<
sin(ωt)
≤
D_{1}
in
index 1
, all switches are turned off by turning off
S_{1}
, which is a low frequency switch. When 
D_{2}
<
sin(ωt)
≤ 
D_{1}
, the dead time is injected into the introduction of the negative half cycle (i.e. 
D_{1}
<
sin(ωt)
≤
0
), with
S_{2}
and
S_{3}
operated at the same time (
Fig. 8
). Similarly, 
D_{1}
and 
D_{2}
are used to inject the dead time at the zero crossing of the grid voltage in
index 2
. When 
D_{2}
<
sin(ωt)
≤ 
D_{1}
in
index 2
, highfrequency switch
S_{3}
is turned off first. When 
D_{1}
<
sin(ωt)
≤
D_{1}
in
index 2
, all switches are turned off by turning off
S_{1}
, which is a lowfrequency switch. The dead time is also injected into the introduction of the positive half cycle (i.e.,
0
<
sin(ωt)
≤
D_{1}
), with
S_{1}
and
S_{4}
operated simultaneously (
Fig. 8
).
The division of energy in
D_{1}
and
D_{2}
is caused by the energy stored in the inductors as explained in subsection A. To minimize the distortion of the current in the next switching state, such stored energy must be eliminated by turning off the bottom switch in advance.
III. PROPOSED ANTIISLANDING METHOD
 A. Conventional AntiIslanding Method in the Topology That Cannot Control the Reactive Power
The topology shown in
Fig. 2
cannot control reactive power as mentioned in Section II.A. Therefore, commonly used conventional antiislanding methods, such as RPV, that vary the reactive power cannot be applied to this topology. In this case, the islanding phenomenon is detected by using active power, which can be controlled by the topology. The typical method that uses active power is the active power variation (APV) method.
The APV method varies the output current reference periodically as depicted in
Fig. 9
(a). In this case, if islanding occurs, islanding detection is realized with the magnitude variation of the PCC voltage. This method only varies the output current reference; it does not vary the frequency and phase of the output current. The APV method also exhibits better characteristics in terms of the THD of the output current in comparison with the methods that vary the frequency of the voltage.
Antiislanding method about active power variation. (a) Magnitude of inverter output current. (b) Inverter output current; (c) Frequency of output voltage.
However, the APV method varies the output current reference periodically (the shape of the output current is shown in
Fig. 9
(b)). When the inverter performs MPPT in this situation, the perturbation and observation (P&O) method is typically used. In the P&O method, the PV cell voltage is repeatedly decreased from the PV open voltage, and the PV cell output power of the previous period is compared with that of the present period to continuously track the maximum power point
[1]
. In this situation, if the active power varies excessively, the PV cell current short circuits, the PV voltage (i.e., the inverter input voltage) suddenly becomes zero (
Fig. 10
), and the inverter stops operation unexpectedly. In sum, the critical drawback of this method is that the system operates precariously depending on the amount of APV.
MPPT operation of PV module.
 B. Basic Principle of AFD Method
The AFD method adds a short period of zero conduction interval into the inverter output current, thus causing a phase difference between the current and the voltage at the PCC as depicted in
Fig. 11
[20]
. The zero conduction interval causes the current injected into the grid by the DG to be slightly distorted (6) such that when islanding occurs, the frequency of the PCC voltage drifts up or down according to (2)
[9]
.
where
f'
is
Inverter output current of AFD method.
Introducing the zero conduction interval
t_{z}
at the end of each half cycle causes the phase angle of the fundamental component of the current to shift. The chopping fraction
cf
is the ratio of the zero conduction interval
t_{z}
to half of the period of the voltage waveform
T_{grid}
.
cf
, defined in (7), determines the difference between the frequency of the inverter output current and the frequency of the grid voltage.
The gridconnected inverter normally operates with a unity power factor. The inverter generates an output voltage that is synchronized with the grid voltage; thus, the frequency of the output voltage is equal to the grid frequency
[9]
. When islanding occurs, the added zero conduction in the current produces a permanent drift in the operating frequency toward the resonance frequency of the local load to maintain a unity power factor. Therefore, the frequency of the PCC voltage increases or decreases because of islanding. This frequency drift eventually reaches the frequency boundary limits to detect islanding
[8]
.
Fig. 12
shows the operational overview of the PCC voltage and current in the AFD method. The local load in
Fig. 1
is divided into resistive load, inductive load, and capacitive load. In the case of resistive load in
Fig. 12
(a), when islanding occurs, the PCC voltage must have the same phase as the fundamental wave of the current reference. In such a case, the frequency of the PCC voltage is higher than that of the PCC voltage under a normal operation. As the voltage leads the current in the case of an inductive load in
Fig. 12
(b), the frequency of the PCC voltage increases and exceeds the resistive load. Consequently, islanding can be detected by checking the increase in frequency under the resistive and inductive loads. However, in the case of capacitive load as depicted in
Fig. 12
(c), the frequency of the PCC voltage can be either increased or decreased. Thus, the islanding phenomenon cannot be detected
[20]
.
AFD operation by load condition. (a) Resistive load. (b) Inductive load. (c) Capacitive load.
 C. Principle of the Proposed Method
The AFD method generates a zero conduction interval, and the zero conduction time injects reactive power into the output current. The conventional AFD method generates a precise sinusoidal wave in the nonzero conduction interval (
Fig. 11
). By contrast, the topology used in the present work cannot produce an output current in the form of a precise sinusoidal wave. Such form can only be realized when the reactive power can be controlled. However, if the switching method in Section II.C is applied to the topology, it easily inject the zero conduction interval into the output current. If the zero conduction interval is injected into the topology by applying the switching method in Section II.C, the output current is obtained as shown in
Fig. 13
. The waveform of the output current in
Fig. 13
does not have the ideal shape shown in
Fig. 11
, but its shape is nonetheless similar to the ideal one. Therefore, the AFD method could be applied to the topology. The injected reactive power could also be obtained by producing the zero conduction interval using a switching method in the topology that cannot control the reactive power.
Normalized inverter output current waveform.
As mentioned in Section III.C, the AFD method detects islanding by checking changes in the frequency of the PCC voltage. As power mismatch generally exists in the PCC voltage (i.e.,
ΔP
≠0,
ΔQ
≠0), the magnitude and frequency of the PCC voltage are changed by this power mismatch when islanding occurs. The relationship between the ratio of the power mismatch and the threshold condition of the magnitude and frequency of the PCC voltage can be expressed as
where
V_{max}
=110%,
V_{min}
=88%,
f
=60Hz,
f_{max}
=60.5Hz, and
f_{min}
=59.3Hz [21]. These values are substituted into (8) and (9) as follows:
As shown in (8) and (9), (9) is related to the frequency. Given that the AFD method is related to the frequency, (9) must be applied to use the AFD method in the topology.
Under a gridconnected operation, the inverter is controlled such that it has the same phase angle for the current and voltage. Therefore, the displacement factor (DPF) of the inverter topology is controlled with unity. The DPF and PF show the following relationship:
where
DF_{i}
is the distortion factor that represents the distortion degree of the current; it can be calculated as
As mentioned previously, the DPF of the inverter is controlled with unity.
PF is the ratio of the active power and the apparent power.
As (15) and (14) are equal, (16) is
Equations (10) and (11) show the NDZ resulting from a power mismatch
[9]
,
[10]
. In the AFD method, increasing the injected reactive power to diminish the NDZ increases the output current THD because
Q
/
P
=THD, as shown in (16)
[10]
. Equation (11) is derived by applying the parameters to (9). If the mismatch of the reactive power lies in this interval, islanding is not detected.
The proposed method involves changing the size of the zero conduction interval injected into the output current every certain period, with the size of the interval not being constant. When the magnitude of the zero conduction interval is increased, the mismatch of the reactive power is set to be sufficiently large to exceed the interval of (11) and thereby eliminate the NDZ. The other zero conduction interval is set to be sufficiently small to reduce the output current THD.
Fig. 14
depicts the conventional AFD method and the proposed method.
Operation of antiislanding method. (a) Conventional AFD. (b) Proposed AFD.
In the proposed method, the switching method explained in section II.C can inject the zero conduction interval. The zero conduction interval can be controlled by
D_{1}
and
D_{2}
.
D_{1}
and
D_{2}
are increased in the cycle of the extended zero conduction interval and decreased in the cycle of the reduced zero conduction interval. In the proposed method,
cf
is increased in the cycle of the extended zero conduction interval. Therefore, the proposed method can control
cf
through
D_{1}
and
D_{2}
.
In sum, the proposed method can be considered to control the output current THD through
D_{1}
and
D_{2}
. Therefore, the proposed method can control the reactive power through (16), which is not related to the active power, because the proposed method perturbs the THD of the output current, assuming that
ΔP
/
P
is nearly zero. If the mismatch of the reactive power lies in the interval of (11) at the moment that the islanding phenomenon occurs, then the mismatch of the reactive power depending on
cf
can be expressed as
If islanding occurs when the proposed method is not applied, then the frequency of the PCC voltage lies in the NDZ (11), and islanding is not detected. Therefore, the proposed method causes a mismatch of reactive power in the output current and must thus make the output current leave the NDZ (11) to detect islanding. In this situation, the proposed method effectively injects reactive power.
The IEEE 1547 standard stipulates that the THD of the inverter output current should not exceed 5%. To satisfy this condition, a relationship must be established between
cf
and the THD of the output current.
Fig. 15
shows the relationship between the THD of the output current and
cf
obtained via simulation. As shown in
Fig. 15
, a proportional relationship exists between the magnitude of
cf
and the THD.
Fig. 15
shows that when
cf
is between 0.045 and 0.046, the THD of the output current is less than 5%. Thus, the range of
cf
that can be maximally used satisfies IEEE 1547.
THDi vs. chopping fraction for the waveform in Fig. 11.
Equation (17) shows the extent to which reactive power is injected into the output current using the proposed method. The proposed method extends the zero current interval every predetermined cycle, as in the analysis in which
cf
is extended periodically. Therefore,
cf
is determined using (17). In the worst case, if the reactive power mismatch
ΔQ
/
P
is 2.37%, which is the lower limit of the NDZ, then
Q_{cf}
/
P
must be injected by 4.02 to cause the output current to leave the NDZ (11). Using (15) and (16),
Q_{cf}
/
P
can be calculated as
As indicated in (18) and
Fig. 15
,
cf
is approximately 0.04 when the THD of the output current is 4.02. If
cf
is 0.046, then islanding can be detected effectively, and the THD of the output current is less than 5%. The zero conduction interval is calculated using
cf
as follows:
D_{1}
is determined via mathematical calculation.
D_{2}
is set to be twice the value of
D_{1}
to consume the energy stored in the inductors on the output side.
IV. SIMULATION RESULTS
A simulation was conducted using PSIM software to validate the proposed method using the circuit configuration in
Fig. 16
. The simulation parameters are listed in
Table I
.
Schematic Circuit of PSIM.
PSIM SIMULATION AND EXPERIMENT PARAMETERS
PSIM SIMULATION AND EXPERIMENT PARAMETERS
The load for the antiislanding test must be designed to completely consume the rated power supplied by the inverter. The procedure of the load design is as follows:
where
P_{load}
is the rated power of the inverter,
V_{out}
is the root mean square voltage of the inverter output phase voltage,
f_{0}
is the resonance frequency, and
Q_{f}
is the quality factor that represents the ratio of the energy lost in one cycle and the average maximum energy stored in the load.
Equation (22) is used to calculate load resistance
R_{load}
and simulate the inverter with a 300 W rated power.
Q_{f}
is set to 1 to calculate the load inductor and capacitor (
L_{load}
and
C_{load}
, respectively), and the resonance frequency is 60 Hz. Using (23) and (24) and these parameters,
L_{load}
and
C_{load}
are calculated. The calculated values of the load are listed in
Table I
.
Thus, the input DC voltage is set to 400 V to supply input DC power to the grid.
Fig. 17
shows the waveform of the output current with the application of
D_{1}
and
D_{2}
, which are calculated in Section III.C.
Fig. 17
(a) shows the case with the expanded zero conduction interval, and
Fig. 17
(b) shows the case with the normal zero conduction interval.
Fig. 18
shows that islanding is detected by applying the proposed method in the case in which the frequency of the PCC voltage is in the normal interval. As mentioned in Section III.B, the frequency indeed increases beyond the normal operating range.
Normalized grid voltage and inverter output current. (a) Extended dead time. (b) Normal dead time.
Islanding detection test for an inverter. (a) PCC voltage, inverter current, fault signal. (b) Frequency of PCC voltage.
V. EXPERIMENTAL RESULTS
The proposed method is validated through experiments conducted with the 300 W gridconnected micro inverter set (
Fig. 19
). The experimental set comprises a control board, sensors, an inverter, and a grid. The RLC load calculated in Section IV is connected to the inverter. The inverter is controlled under current control mode by employing a STM32F405RG in the ARMCortex M4 family micro controller unit (MCU). The proposed antiislanding algorithms are implemented in the MCU. In this experiment, the MCU generates 50 kHz PWM gate signals to implement a proportional–integral (PI) controller, which is a typical controller for singlephase AC systems.
Experimental setup.
Fig. 20
shows the waveform of the switch operation. In
Fig. 20
(b), the lower switch
S_{4}
is turned off in advance, then
S_{1}
is turned off to provide the freewheeling path mentioned in Section II.C. As a result, the dead time in which all the switches are turned off is formed after
S_{1}
is turned off and before
S_{2}
is turned on.
Waveform of switch operation (experimental waveform). (a) Waveform of switch operation (ordinary). (b) Waveform of switch operation (enlarged).
Fig. 21
shows the waveform of the proposed method applied to the inverter before the grid is disconnected. The zero conduction interval of the output current is extended periodically. In the experiment, the zero conduction interval of the output current is extended every 10 cycles of the grid voltage. When the grid is connected, the magnitude and frequency of the PCC voltage are maintained. However, the THD in the current is increased because of the extended zero conduction interval in the proposed method. A YOKOGAWA WT3000 power analyzer is used to measure the THD of the grid voltage and the output current, which are 0.104% and 3.351%, respectively.
Output waveform using proposed method before islanding (experimental waveform).
Fig. 22
shows that islanding is detected using the proposed method. If the grid is disconnected at an arbitrary time, islanding occurs until the output current enters the extended zero conduction cycle. The islanding phenomenon is detected in the extended zero conduction cycle. Therefore, once islanding occurs, the magnitude and frequency of the PCC voltage fall in the normal operating range. The frequency of the PCC voltage is then increased until it is out of the normal operating range in the extended zero conduction cycle. Finally, the inverter is shut down.
Fig. 22
shows the procedure for detecting the islanding phenomenon with this process.
Antiislanding test result for proposed method (experimental waveform).
The experimental results in
Figs. 21
and
22
match the simulation results of
Figs. 17
and
18
, respectively.
VI. CONCLUSIONS
This study proposes an antiislanding method that is based on an application of the AFD method that periodically perturbs the zero conduction interval in an inverter topology that cannot control the reactive power. If the proposed method is applied to the said topology, the reactive power can be controlled indirectly by using the switching method and perturbing the output current. Thus, the islanding phenomenon can be detected by varying the reactive power. The simulation and experimental results demonstrate that the proposed method can effectively detect islanding in the inverter topology used in this work.
Acknowledgements
This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2016R1A2B4010636) and the Human Resources Program in Energy Technology through the Korea Institute of Energy Technology Evaluation and Planning (KETEP), which was granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20134030200310)
BIO
Raeho Kwak received his B.S. degree in Electrical and Computer Engineering from Ajou University, Korea, in 2015. He is currently working toward his M.S. degree at Ajou University, Korea. His research interests include gridconnected systems, threelevel inverter systems, and DC–DC converter systems. JuneHee Lee was born in Korea. He received his B.S. degree in Electronic Engineering from Ajou University, Korea, in 2013. He is currently working toward his Ph.D. degree in Electronic Engineering at Ajou University, Korea. His research interests include power conversion and gridconnected systems.
JuneHee Lee was born in Korea. He received his B.S. degree in Electronic Engineering from Ajou University, Korea, in 2013. He is currently working toward his Ph.D. degree in Electronic Engineering at Ajou University, Korea. His research interests include power conversion and gridconnected systems.
KyoBeum Lee received his B.S. and M.S. degrees in Electrical and Electronic Engineering from Ajou University, Korea, in 1997 and 1999, respectively. He received his Ph.D. degree in Electrical Engineering from Korea University, Korea, in 2003. From 2003 to 2006, he worked with the Institute of Energy Technology, Aalborg University, Aalborg, Denmark. From 2006 to 2007, he worked with the Division of Electronics and Information Engineering, Chonbuk National University, Jeonju, Korea. In 2007, he joined the School of Electrical and Computer Engineering, Ajou University, Suwon, Korea. His research interests include electric machine drives, renewable power generation, and electric vehicle applications. He is an associate editor of the IEEE Transactions on Power Electronics, the Journal of Power Electronics, and the Journal of Electrical Engineering & Technology.
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