Current control of grid-connected inverter using integral sliding mode control and resonant compensation

International Journal of Power Electronics and Drive System (IJPEDS)
Vol. 10, No. 2, June 2019, pp. 1022~1033
ISSN: 2088-8694, DOI: 10.11591/ijpeds.v10.i2.pp1022-1033  1022
Journal homepage: http://iaescore.com/journals/index.php/IJPEDS
Current control of grid-connected inverter using integral sliding
mode control and resonant compensation
Seung-Jin Yoon, Thanh Van Nguyen, Kyeong-Hwa Kim
Department of Electrical and Information Engineering, Seoul National University of Science and Technology, Korea
Article Info ABSTRACT
Article history:
Received Nov 17, 2018
Revised Jan 22, 2019
Accepted Mar 7, 2019
To eliminate the adverse effect from parameter variations as well as distorted
grid conditions, a current control scheme of an LCL-filtered grid-connected
inverter using a discrete Integral Sliding Mode Control (ISMC) and resonant
compensation is presented. The proposed scheme is constructed based on the
cascaded multiloop structure, in which three control loops are composed of
grid-side current control, capacitor voltage control, and inverter-side current
control. An active damping to suppress the resonance caused by LCL filter
can be effectively realized by means of the inverter-side feedback control
loop. Furthermore, the seamless transfer operation between the grid-
connected mode and islanded mode is achieved by the capacitor voltage
control loop. To retain a high tracking performance and robustness of the
ISMC as well as an excellent harmonic compensation capability of the
Resonant Control (RC) scheme at the same time, two control methods are
combined in the proposed current controller. As a result, the proposed
scheme yields a high quality of the injected grid currents and fast dynamic
response even under distorted grid conditions. Furthermore, to reduce the
number of sensors, a discrete-time reduced-order state observer is introduced.
Simulation and experimental results are presented to demonstrate the
effectiveness of the proposed scheme.
Keywords:
Distributed generation
Grid-connected inverter
Indirect current control
Integral sliding mode control
Power quality
Copyright © 2019 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
Kyeong-Hwa Kim,
Department of Electrical and Information Engineering,
Seoul National University of Science and Technology,
232 Gongneung-ro, Nowon-gu, Seoul, 01811, Korea.
Email: k2h1@seoultech.ac.kr
1. INTRODUCTION
In recent years, renewable energy sources (RESs) such as wind and solar are attracting great
attentions due to the depletion of fossil energy and environmental issues [1-3]. To integrate various RESs
into the utility grid, Distributed Generation (DG) systems as a component of microgrid (MG) has been
developed [4]. With the advantages of easy integration of resources, flexible installation location, and reliable
operation, DG-based MG becomes a future trend of constructing the electric power system [5]. Basically,
MG is formed by connecting DG units with energy storage system and local loads. To interface DG units
with the utility grid, the Interlinking Converters (ICs) are usually employed. Currently, most of ICs adopt a
Voltage Source Inverter (VSI) with current control to regulate the current injected into the grid. The grid-
connected VSI should be controlled to achieve a high performance of injected grid currents in the sense of
fast dynamic response, robustness to perturbation, zero tracking error, and low Total Harmonic
Distortion (THD).
With the aim of attenuating the harmonic components caused by the pulse width modulation
(PWM), the filter is connected between the utility grid and VSI [6]. In recent years, LCL filters are widely
applied to a grid-connected inverter system because they provide a better harmonic attenuation capability [7].
Moreover, LCL filter has lower cost and smaller physical size in comparison to L filter to achieve the same

Int J Pow Elec & Dri Syst, ISSN: 2088-8694 
Current control of grid-connected inverter using integral sliding mode control and… (Seung-Jin Yoon)
1023
harmonic attenuation. Due to this advantage, the study of LCL filter has been a topic of intense research
mainly in terms of design and development of control methods. However, because the LCL filter causes the
resonance problem, a current control strategy of an LCL-filtered grid-connected inverter should be carefully
designed to stabilize the system.
The resonance problem of LCL filter can be dealt with by two main methods, namely, the passive
damping and active damping. The passive damping is simply achieved by adding a resistor in series with the
filter capacitor [8]. Despite simple implementation, this method causes extra losses through heat dissipation,
resulting in reduced system efficiency. To overcome such a limitation, the active damping technique which
damps the resonance by a control algorithm has been introduced. To stabilize an inverter system without
increasing the loss, an active damping approach based on a virtual resistance has been studied by using the
filter capacitance current feedback [9]. However, this scheme requires extra sensors to measure the capacitor
currents, which leads to the cost increase and hardware complexity of system.
To provide uninterruptable power to the critical load even in the occurrence of utility outage, the grid-
connected inverter should operate in both the grid-connected mode and islanded mode [10]. In the grid-
connected mode, the inverter is operated with current-controlled mode to inject the power into the utility grid,
while the critical load voltage is maintained by the grid voltage. Once the utility fault occurs, the inverter
operation is switched into the islanded mode, in which the inverter is operated with the voltage-controlled
mode. The transition between these two control modes should be seamless to prevent the load damage due to
unstable load voltage. For this purpose, a multiloop indirect current control method has been presented as one
of the effective approaches. The seamless transfer is achieved by combining the outer control loop of grid-
side current and inner control loop of filter capacitor voltage in [11]. By controlling the capacitor voltage in
both the grid-connected and islanded modes, this method is capable of supplying the critical load with a
stable and seamless voltage during the whole period. However, the control design is quite complicated.
Furthermore, this scheme requires extra sensing devices to implement additional control loops. In addition,
the adverse grid voltages such as the harmonic distortion have not been addressed.
To control the grid-connected inverter, the proportional-integral (PI) decoupling control has been
widely used with the voltage feedforward because of its simplicity and stability. However, since the
conventional PI decoupling control inherently has a poor disturbance rejection capability, it is not a suitable
way to control grid-connected inverters under grid voltage perturbed by disturbances. To improve the power
quality of inverter system, several studies have been investigated regarding the harmonic compensation
strategy. The harmonic compensation can be achieved by two methods, namely, selective and non-selective
schemes. As a selective harmonic compensation scheme, the proportional-resonant (PR) controller is widely
employed because it provides a good reference tracking as well as harmonic compensation [12]. However, as
the compensated harmonic terms are increased, the computational burden is also increased due to the
implementation of separate resonant controllers.
As a non-selective harmonic compensation strategy, the predictive control [13] and repetitive
control [14] schemes have been studied. Although the predictive control has a capability of providing fast
dynamic response with minimum THD level, it is very sensitive to system parameter change. The repetitive
control provides a good harmonic rejection for nonlinear loads with periodic distortions. However, slow
dynamic response is the main shortcoming of this type of controller due to the learning process. To overcome
the drawbacks of the aforementioned approaches, a nonlinear control strategy such as a sliding mode control
(SMC) has been presented. Since the SMC is inherently insensitive to the presence of parameter uncertainties
and external disturbances, many studies have been carried out to apply the SMC technique to grid-connected
inverters [15-17]. A SMC scheme based on dual-loop control structure is presented in [15]. This approach not
only guarantees the system stability, but also alleviates the chattering phenomenon effectively. In order to
enhance the tracking performance and reduce the THD of injected grid currents effectively, a SMC scheme
combined with multiple resonant terms is proposed in [16]. However, the implementation of this scheme is
complicated in practice since it employs many resonant terms in the sliding surface function. Moreover, this
approach requires several sensing devices to measure the inverter-side currents and capacitor voltages, which
increases the system cost and hardware complexity. To eliminate the steady-state error, an integral sliding
mode control (ISMC) is designed by using the continuous-time model [17], in which the integral term is
added into the sliding function. For easiness in implementation by digital controller, a discrete SMC
approach is developed for grid-connected inverters [18], [19].
This paper presents a current control scheme for a three-phase LCL-filtered grid-connected inverter
based on a discrete integral sliding mode control (ISMC) combined with resonant compensation. The
proposed control scheme is constructed by using the multiloop indirect control structure which is composed
of the grid-side current control loop, capacitor voltage control loop, and inverter-side current control loop. By
adding the capacitor voltage control loop into the control structure, the seamless transfer between the grid-
connected mode and islanded mode can be achieved. In addition, a discrete-time reduced-order state observer

 ISSN: 2088-8694
Int J Pow Elec & Dri Syst, Vol. 10, No. 2, June 2019 : 1022 – 1033
1024
is introduced with the aim of reducing the total number of sensing devices. In the proposed control scheme,
the active damping is realized through a feedback control loop of inverter-side current by using the estimated
inverter-side current from the state observer. As a result, the proposed control scheme not only ensures the
stability by active damping but also reduces the system cost since the use of additional sensing devices can be
avoided. With the motivation to improve the current quality injected into the grid under harmonically
distorted grid voltage, the resonant controllers (RCs) are combined with the discrete ISMC in outer-loop grid-
side current controller in the discrete-time domain. While the robustness and fast dynamic response of grid
current control are ensured by the discrete ISMC, a high quality of current with low THD is achieved by the
harmonic compensation based on the RC. To verify the effectiveness and validity of the proposed current
control scheme, the PSIM software-based simulations and experiments have been carried out by using three-
phase 2 kVA prototype grid-connected inverter. The entire control algorithm is implemented on a digital
signal processor (DSP) TMS320F28335 and tested under distorted grid conditions. This paper is organized as
follows. Section 2 presents the modeling of inverter. Section 3 explains the proposed discrete ISMC scheme
with the harmonic compensation. In section 4, the simulations and experimental results are presented.
Finally, this paper is concluded in Section 5.
2. MODELING OF LCL-FILTERED GRID-CONNECTED INVERTER
Figure 1 shows a configuration of a three-phase grid-connected inverter connected to the grid
through an LCL filter, in which VDC represents the DC link voltage, R1 and R2 are the filter resistances, L1 and
L2 are the filter inductances, and Cf is the filter capacitance. The state equations of the grid-connected inverter
are expressed in the synchronous reference frame (SRF) as follows:
Figure 1. Configuration of three-phase grid-connected inverter with LCL filter
2 2
2 2 2 2
2 2
2 2 2 2
/ 1/ 0 1/ 0
/ 0 1/ 0 1/
q q cq q
d d cd d
i i v e
R L L L
d
i i v e
R L L L
dt
  
       
     
     
       
     
  
     
       
(1)
1 1
1 1 1 1
1 1
1 1 1 1
/ 1/ 0 1/ 0
/ 0 1/ 0 1/
q q cq iq
d d cd id
i i v v
R L L L
d
i i v v
R L L L
dt
  
       
     
     
       
     
  
     
       
(2)
2 1
2 1
1/ 0
0 1/ 0
0 1/
0 0 1/
f
cq cq q q
f
f
cd cd d d
f
C
v v i i
C
d
C
v v i i
C
dt
 

  
       
 
     
 
 
       
  

   
       
   
(3)
where the subscripts “q” and “d” denote the variables in the SRF, 2 2
[ ]T
q d
i i is the grid-side current vector,
1 1
[ ]T
q d
i i is the inverter-side current vector, [ ]T
cq cd
v v is the capacitor voltage vector, [ ]T
q d
e e is the grid
voltage vector, [ ]T
iq id
v v is the inverter output voltage, and ω is the angular frequency of the grid voltage.
From (1) to (3), the continuous-time model of inverter system can be expressed in the SRF as;
( ) ( ) ( ) ( )
t t t t
  
x Ax Bu De
 (4)

1025
( ) ( )
t t

y Cx (5)
where 2 2 1 1
[ ]T
q d q d cq cd
i i i i v v

x , [ ]T
iq id
v v

u , [ ]T
q d
e e

e , and the system matrices A, B, C and D are
expressed as
2 2 2
2 2 2
1 1 1
1 1 1
/ 0 0 1/ 0
/ 0 0 0 1/
0 0 / 1/ 0
0 0 / 0 1/
1/ 0 1/ 0 0
0 1/ 0 1/ 0
f f
f f
R L L
R L L
R L L
R L L
C C
C C
 
 
 
 
 
 
  
 
  
 
 
 
 
 
 
 
A =
1
1
0 0
0 0
1/ 0
0 1/
0 0
0 0
L
L
 
 
 
 
 
 
 
 
 
 
B = ,
2
2
1/ 0
0 1/
0 0
0 0
0 0
0 0
L
L

 
 

 
 
 
 
 
 
 
 
D = ,
1 0 0 0 0 0
0 1 0 0 0 0
 
 
 
C = .
For a digital implementation, the continuous-time model of the inverter system
can be discretized as
( 1) ( ) ( ) ( )
d d d
k k k k
   
x A x B u D e (6)
( ) ( )
d
k k

y C x (7)
where the matrices d
A , d
B , d
C , and d
D can be calculated with the sampling period s
T as
2 2
1! 2!
s
T s s
d
T T
e
    
A A A
A I  (8)
1
( )
d d

 
B A A I B , d 
C C (9)
1
( )
d d

 
D A A I D . (10)
3. PROPOSED DISCRETE SLIDING MODE CONTROL WITH HARMONIC COMPENSATION
Figure 2 shows the overall block diagram of the proposed control scheme for an LCL-filtered three-
phase grid-connected inverter. The proposed control scheme mainly consists of an ISMC, resonant
compensation, and reduced-order observer in the discrete-time domain. The inverter reference voltages by
the proposed current controller are applied through the space vector PWM. The proposed control scheme
requires the measurement of only the grid-side currents and grid voltages. The measured grid voltages are
used in the phase-locked loop (PLL) scheme to determine the grid phase angle.

 ISSN: 2088-8694
1026
Figure 2. LCL-filtered three-phase grid-connected inverter and the proposed control scheme
Figure 3 shows the proposed current control scheme for the q-axis current based on the multiloop
structure. To realize a cascaded multiloop controller, all the system states should be available for feedback
purpose. However, additional sensing devices usually increase the total cost and hardware complexity in a
practical inverter system. Therefore, a discrete-time reduced-order state observer is employed to obtain the
estimated signals for the inverter-side currents and the capacitor voltages in the proposed control scheme.
The discrete-time inverter model in (6) and (7) can be rewritten as
Figure 3. Proposed current control scheme; q-axis current control
1 11 12 1 1 1
2 21 22 2 2 2
( 1) ( ) ( ) ( )
( ) ( )
( 1) ( ) ( ) ( )
k k k k
k k
k k k k

         
  
         

         
x A A x B D
u e
x A A x B D
(11)
  1
1 2
2
( )
( )
( )
k
k
k
 
  
 
x
y C C
x
(12)
where 1 2 2
[ ]T
q d
i i

x and 2 1 1
[ ]T
q d cq cd
i i v v

x . Then, the estimated state variables can be obtained by
applying the discrete-time reduced-order state observer as [20]
2 1
ˆ ( ) ( ) ( )
k k k
 
x η Kx (13)
22 12 22 12 21 11
2 1 2 1
( 1) ( ) ( ) ( ) ( ) ( ) ( )
( ) ( ) ( ) ( )
k k k k
k k
      
   
η A KA η A KA Ky A KA y
B KB u D KD e
(14)

1027
where the symbol “ˆ” denotes the estimated variables and K is the observer gain matrix. The estimated
variables 2
x̂ are used in the multiloop control structure in Figure 3 instead of actual measurements.
To control the grid-side currents with a good disturbance rejection, an ISMC and resonant
compensation are designed in the discrete-time domain. From (1), the state model of a grid-side current is
expressed in the SRF as follows:
1 1
( ) g g c g
t   

x A x B v D e (15)
where [ ]T
c cq cd
v v

v
2
2
/
/
g
R L
R L
 
 
  
 
 
A ,
2
2
1/ 0
0 1/
g
L
L
 
  
 
B ,
2
2
1/ 0
0 1/
g
L
L

 
  

 
D . (16)
The continuous-time model of grid-side current can be similarly discretized by using (8) - (10) as
1 1
( 1) ( ) ( ) ( ) ( )
dg dg c dg
k k k k k
   
x A x B v D e . (17)
Let us define the error state variable E(k) as
*
1 1
( ) ( ) ( )
k k k
 
E x x (18)
where *
1 ( )
k
x is the reference grid-side current vector. To eliminate the steady-state error, an integral sliding
surface function S(t) is selected as
( ) ( ) ( )
I
k
t t t
s
 
S E E (19)
where kI is the integral gain. This integral sliding surface can be discretized by using the relation as
1
1
2(1 )
(1 )
s
z
s
T z





. (20)
Then, the integral sliding surface function can be expressed in the discrete-time domain as follows:
( 1) ( 1) ( ) ( 1) ( ) ( )
2 2
I s I s
k T k T
k k k k k k
       
S E E E E S . (21)
In general, the SMC suffers from the chattering phenomenon that causes current harmonics,
increases power losses, and also produces severe electromagnetic compatibility noise in inverter applications.
In order to eliminate such a chattering problem, the study in [21] gave the complete definition and detailed
physical explanation for the quasi-sliding mode, in which a reaching condition is expressed as follows:
( 1) ( ) ( ) sgn( ( ))
s s
k k qT k T k

   
S S S S , 0
q  , 0
  . (22)
The control input of the ISMC can be obtained as
*
eq L N
  
u u u u (23)
where eq
u is the equivalent control, L
u is the linear control, and N
u is the nonlinear switching control. From
(15), (18), (21), and (22), these control inputs can be obtained as follows:
1 *
1 1
2
( ) ( ) ( ) ( )
2
I s
eq dg dg dg
I s
k T
k k k k
k T
  
 
   
 

 
u B x A x D e E (24)

 ISSN: 2088-8694
1028
1 2
( )
2
L dg s
I s
qT k
k T
  
  

 
u B S (25)
1 2
sgn( ( ))
2
N dg s
I s
T k
k T

  
  

 
u B S . (26)
The proposed scheme also employs the resonant controllers of grid-side current for harmonic
compensation. The resonant controllers are implemented in the SRF to take advantage of the simultaneous
compensation for both the negative and positive sequence harmonics by one resonant term. Thus, the
harmonically distorted grid at the 5th, 7th, 11th and 13th can be suppressed by compensating the harmonic
components at 6th and 12th in the SRF. The transfer function of resonant controller is expressed as
2 2 2
6,12
( )
( )
I h
h
s
R s K
s h


 
 (27)
where h denotes the harmonic order and h
K is the resonant gain. The transfer function of the resonant
controller in (27) is implemented in the discrete-time domain by using the impulse invariant method. As
shown in Figure 3, the grid-side current control loop generates the filter capacitor reference voltages
*
c
v for
the q- and d- axes. For the inner-loop capacitor voltage control and inverter-side current control, the PI
controllers are employed with the estimated states.
4. SIMULATION AND EXPERIMENTAL RESULTS
In order to verify the feasibility and validity of the proposed current control scheme, the simulations
have been carried out for an LCL-filtered three-phase grid-connected inverter based on the PSIM software.
The system configuration and the proposed control scheme are depicted in Figure 2 and Figure 3. The system
parameters are listed in Table 1.
Table 1. System parameters of a grid-connected inverter
Parameter Value
DC link voltage 420 V
Grid voltage 220 V
Inverter-side filter inductance 1.7 mH
Grid-side filter inductance 0.9 mH
Filter capacitance 4.5 µF
Filter resistance 0.5 Ω
Load bank 24 Ω
Grid frequency 60 Hz
Switching frequency 10 kHz
Figure 4 shows the simulation results of the proposed current control scheme at steady-state under
distorted grid condition. Figure 4(a) represents three-phase distorted grid voltages used for the simulations.
The abnormal grid voltages contain the 5th, 7th, 11th, and 13th harmonic orders with the magnitude of 5% of
the fundamental component. Figure 4(b) through Figure 4(d) shows the simulation results of using the
proposed current control under the distorted grid voltages as in Figure 4(a). The reference value of the grid-
side current is set to 7 A. Even though the grid voltages are severely distorted, the proposed control scheme
provides a good harmonic compensation capability as can be observed from Figure 4(b), giving quite
sinusoidal phase currents without noticeable distortion. Figure 4(c) shows the steady-state responses of the q-
axis and d-axis grid-side currents at the SRF. As is shown, the grid-side currents track the reference values
well, which indicates a good reference tracking performance of the proposed scheme. To verify the quality of
injected grid currents, Figure 4(d) shows the Fast Fourier Transform (FFT) result for grid-side a-phase
current. This figure also includes the harmonic limits specified by the grid interconnection regulation IEEE
Std. 1547 [22]. As can be seen clearly, the harmonic components in the 5th, 7th, 11th, and 13th orders of the
injected grid currents can be effectively damped by the proposed control scheme. The THD value of current
is only 3.36%, which meets the quality criteria.

1029
(a) (b)
(c) (d)
Figure 4. Simulation results for steady-state responses under distorted grid voltages with the proposed current
control scheme (a) three-phase distorted grid voltages, (b) three-phase grid-side current responses, (c) grid-
side current responses at the SRF, (d) FFT result for grid-side a-phase current
To demonstrate the transient performance of the proposed current control scheme, Figure 5 shows
the simulation results under the same distorted grid condition as in Figure 4(a) when the q-axis grid-side
current reference has a step change from 6 A to 10 A at 0.04 s, and again, from 10 A to 6 A at 0.07 s. Figure
5(a) and Figure 5(b) represent the transient responses of the grid-side currents at the natural reference frame
and SRF, respectively. It can be clearly observed from these figures, except for small current oscillations
during transient periods, the actual currents track the references rapidly and immediately reach new steady-
state values. This indicates a good and fast transient response of the proposed control scheme.
(a) (b)
Figure 5. Simulation results for transient responses under distorted grid voltages with the proposed current
control scheme, (a) three-phase grid-side current responses, (b) grid-side current responses at the SRF
Figure 6 shows the simulation results for the inverter states and estimated states by using the
discrete-time reduced-order state observer at the SRF. Obviously, the estimated states can follow the actual
ones even under harmonic injection condition, which confirms a stable operation of the observer.
Figure 7 shows the simulation results for the estimated three-phase inverter-side currents and
capacitor voltages at the natural reference frame. These estimation results clearly demonstrate a good
estimating performance of the observer.
To verify the effectiveness of the proposed control scheme in practice, the experiments have been
carried out. Figure 8 shows the configuration of the experimental system based on DSP TMS320F28335.

 ISSN: 2088-8694
1030
Figure 9 shows three-phase distorted grid voltages used for experimental evaluations. These grid
voltages contain the 5th, 7th, 11th, and 13th harmonic orders with the magnitude of 5% of the fundamental
component as the simulation in Figure 4(a).
(a) Inverter-side currents and estimated states at
the SRF
(b) Capacitor voltages and estimated states at the
SRF
Figure 6. Simulation results for the estimating performance of the discrete-time reduced-order state observer,
(a) Inverter-side currents and estimated states at the SRF, (b) Capacitor voltages and estimated states at the
SRF
(a) (b)
Figure 7. Simulation results for estimated three-phase states, (a) estimated three-phase inverter-side currents,
(b) estimated three-phase capacitor voltages
Figure 8. Configuration of the experimental system Figure 9. Three-phase distorted grid voltages used in
experiments
Figure 10 shows the experimental results of the proposed current control scheme in terms of steady-
state and transient responses under the distorted grid condition. It can be seen from Figure 10(a), the
proposed control scheme provides considerably sinusoidal three-phase current waveforms at steady-state,
which is consistent well with the simulation result in Figure 4(b). To evaluate the transient performance of
the proposed control scheme, a step change in grid-side current reference from 4 A to 6 A is introduced. It
can be observed from Figure 10(b) that the grid-side currents can track new references rapidly, which is also

1031
matched with the behavior as shown in Figure 5(a). This confirms a good dynamic and steady-state
performance of the proposed current control scheme in the presence of severely distorted condition of
grid voltages.
Figure 11 shows the experimental results for the estimating performance of the discrete-time
reduced-order state observe. In these figures, the estimated three-phase inverter-side currents and capacitor
voltages are calculated in a DSP by using the estimated states at the SRF. Clearly, the experimental results
are compatible with the simulation results in Figure 7, showing a good estimating performance.
(a) (b)
Figure 10. Experimental results for the proposed current control scheme under distorted grid voltages, (a)
steady-state response, (b) transient response
(a) (b)
Figure 11. Experimental results for the estimating performance of discrete-time reduced-order state observer,
(a) Estimated three-phase inverter-side currents, (b) Estimated three-phase capacitor voltages
5. CONCLUSIONS
This paper has presented a multiloop indirect current control scheme for a three-phase LCL-filtered
grid-connected inverter based on a discrete ISMC scheme and a resonant compensation. The proposed
scheme is constructed by three cascaded control loops consisting of a grid-side current control, a capacitor
voltage control, and an inverter-side current control. In general, to implement the multiloop current control
scheme, many sensing devices should be required, which increases the cost and complexity of system. To
overcome this limitation, a discrete-time reduced-order state observer is used to estimate both the inverter-
side currents and capacitor voltages by using only the measured grid-side currents and grid voltages. This
practically reduces the cost and complexity of an LCL-filtered grid-connected inverter system. For the
purpose of suppressing the resonance behavior due to LCL filter, an active damping has been realized by
means of the inverter-side current feedback loop. This approach is more feasible in comparison with the filter
capacitor current feedback since the inverter-side current can be easily estimated by state observer. By
integrating the capacitor voltage control loop into the control structure, the seamless transfer between the
grid-connected mode and islanded mode is effectively achieved. To retain a high tracking performance and

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robustness of the ISMC as well as an excellent harmonic compensation capability of the RC at the same time,
these two control methods are combined in the proposed current control scheme. As a result, fast dynamic
response, zero steady-state error, and robustness against the system uncertainties can be ensured by the
proposed control. To verify the usefulness of the proposed control, a 2 kW prototype grid-connected inverter
has been constructed and the whole control algorithm has been implemented on 32-bit floating-point DSP
TMS320F28335. Comprehensive simulation and experimental results have been presented under distorted
grid environment to demonstrate the usefulness of the proposed current control scheme.
ACKNOWLEDGEMENTS
This study was supported by the Research Program funded by the SeoulTech (Seoul National
University of Science and Technology).
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BIOGRAPHIES OF AUTHORS
Seung-Jin Yoon was born in Seoul, Korea, in 1989. He received the M.S. degree in Electrical
and Information Engineering from Seoul National University of Science and Technology,
Seoul, Korea, in 2018. Currently he is working toward the Ph.D. degree in the Department of
Electrical and Information Engineering at Seoul National University of Science and
Technology. His research interests include renewable energy and power electronics.
Thanh Van Nguyen was born in Hai Duong, Vietnam, in 1994. He received the B. Eng. degree
in Control and Automation Engineering from Hanoi University of Science and Technology,
Hanoi, Vietnam, in 2017. Currently he is working toward the M.S. degree in the Department of
Electrical and Information Engineering at Seoul National University of Science and
Technology. His research interests include renewable energy and power electronics.
Kyeong-Hwa Kim was born in Seoul, Korea, in 1969. He received his B.S. degree from
Hanyang University, Seoul, Korea, in 1991; and his M.S. and Ph.D. degrees from the Korea
Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 1993 and 1998,
respectively, all in Electrical Engineering. From 1998 to 2000, he was a Research Engineer
with Samsung Electronics Company, Korea, where he was engaged in the research and
development of AC machine drive systems. From 2000 to 2002, he was a Research Professor
with KAIST. From August 2010 to August 2011, he was a Visiting Scholar with the Virginia
Polytechnic Institute and State University (Virginia Tech), Blacksburg, VA, USA. Since
August 2002, he has been with the Seoul National University of Science and Technology,
Seoul, Korea, where he is presently working as a Professor. His current research interests
include AC machine drives, the control and diagnosis of power systems, power electronics,
renewable energy, and DSP-based control applications. Professor Kim is a Member of the
Korean Institute of Power Electronics (KIPE) and IEEE.

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