vol. 4(12) issn © austrian e-journals of universal scientific...

International Journal of Mechatronics, Electrical and Computer Technology

Vol. 4(12), Jul, 2014, pp. 1297-1327, ISSN: 2305-0543

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© Austrian E-Journals of Universal Scientific Organization

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1297

A Flexible Control Method for Integration of Distributed

Generation Resources to the Power Grid in Presence of Nonlinear

Load and Unbalanced Voltage

Hamed Tahami1*

, Seyed Mehdi Hosseini2 and Jafar Adabi

3

1,2,3Babol Noshirvani University of Technology, Babol, Iran

*Corresponding Author's E-mail:[email protected]

Abstract

This paper deals with a converter control strategy for the integration of DG (Distributed

Generation) to the power grid in presence of nonlinear load and unbalanced voltage

condition. The proposed control scheme based on instantaneous power theory is

investigated in the αβ stationary frame. This control scheme controls the current harmonic

flow between the grid and the inverter in a way that the current harmonic distortion rate at

the Point of Common Coupling (PCC) is compatible to IEEE standard. Simulation results

using MATLAB/SIMULINK are presented for a 100 KW DG connected to the distribution

grid via an inverter to confirm the validity and effectiveness of the proposed control

scheme. Furthermore, a wind turbine unit, regarded as a DG unit, is simulated to evaluate

the dynamic impact of distributed generation. Additionally, the reactive power

compensation and the grid power factor correction are also achieved using this procedure.

Keywords: DG (Distributed Generation) resources, grid-connected inverter, nonlinear

loads, unbalanced voltage condition, stationary reference frame

1. Introduction

In recent years, due to the importance of environmental pollution and the termination of

fossil fuels, DG (Distributed Generation) technology has been considered for feeding local

loads or delivering power to medium and low voltage grid [1].Moreover, electrical energy

generation, at the consumption place, reduces the transmission line losses and enhances the


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grid reliability [2]. As most of different types of DG resources such as wind turbines,

photovoltaic, fuel-cells, etc. are connected to the grid using power electronic devices, their

design, control, and power quality are of great importance [3].Power electronic converters

are proposed for interfacing between DG and distribution grids. By developing power

electronic devices, nonlinear loads in the grid are increased and current harmonic

distortion rate is raised extremely. Moreover, in large scale three-phase networks, the grid

can be unbalanced at any moment. The connection of inverters to the grid and its control

procedure are challenging in harmonic and unbalanced environment. As a result, it is

necessary to design an appropriate control method for power electronic interface circuits in

such conditions [4]. A robust control strategy to regulate the grid current entering a

distribution network from a three-phase VSI system connected via a LCL filter has been

proposed in [5]. This strategy has integrated an outer loop grid current regulator with inner

capacitor current regulation to stabilize the system. A synchronous frame PI current

regulation strategy has been used for the outer grid current control loop. A SVPWM-based

predictive current controller has been proposed in [6] to effectively compensate for grid

harmonics and to completely eliminate the effect of control delay using software predictors

and filters based on a dual-timer sampling system. The controller has operated in the

synchronous d-q reference frame and mimicked the dead-beat control to control the output

current to achieve its reference in each PWM period. A review of linear and nonlinear

current control techniques has been presented in [7] with a brief description of their

advantages and disadvantages. The application field where each technique is

particularly suited has been also indicated. In [8] a nonlinear control technique of

three-phase voltage source shunt active power filter has been proposed. In this model,

the currents injected by the active filter are controlled in the synchronous d-q reference

frame using decoupled nonlinear control strategy.

In this paper, the multifunctional control approach for VSI connected to the grid

through LCL filter is proposed to control the harmonic loads at PCC. The goal is to


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eliminate the harmonic current injection at the grid-connected mode. In application of

DG resources, system’s operating point varies due to environmental conditions. Therefore,

it is necessary to tune PLL parameters versus operating point variation. Moreover, because

of the PLL nonlinear characteristics and implementation difficulties especially in

harmonic conditions, the reference frame-based control method is proposed instead

of using PLL.To compensate for the harmonic currents, the PR controller is suggested

and the pole-zero cancellation method is used in order to tune controller parameters .

This paper is structured as follows. The next section will describe the system

structure and the inverter output filter design. It will be followed by the section in

which the control scheme that controls active and reactive power based on

instantaneous power theory is illustrated. Then, the performance of the proposed

control scheme will be validated by means of two simulations: firstly, the converter

interfaces a controlled current source and secondly, it interfaces a wind power

generation system to a distribution grid supplying nonlinear loads in unbalanced

voltage conditions. Finally, the conclusion will be drawn.

2. System Structure

The system under analysis is composed of an AC grid, a nonlinear load and a renewable

energy source interfaced to the grid by means of an inverter with LCL filter. The energy

source may be a DC-power source or an AC source which is rectified into DC. Due to this,

the DG source is modeled with a DC controlled current source. To show its dynamic

behavior as regards the voltage variation, the current source is paralleled with a capacitor.

Figure 1 shows the system configuration. The considered nonlinear load is a full-wave

diode converter which supplies a resistive load in each phase.


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dcv

dcL

aI

bI

cI

a

b

c

av

bv

cv

abcLI

Output Filter

Rs Ls ACLgLf

Cf

RL

switch

C

Figure1: System configuration of three-phase grid-connected inverter with LCL filter

2.1. Inverter output filter

The inverter output filter plays two major roles: (1) eliminating harmonics caused by

switching of the output current; (2) protecting inverter from transient states. The proposed

inverter output filter is designed on the base of its first role. For adjusting the injected

current to the grid according to electrical power quality standard, it is necessary to install a

harmonic filter in the inverter output.

2.2. LCL filter design

The LCL filter components which are constructed from two inductors and a capacitor

bank are selected in a way that the current injection must be adjusted to the standard. Table

1 shows IEEE standard for the quality of current injection to the grid in PCC.


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Table 1: Current distortion limitation in PCC according to IEEE standard [9]

Current Distortion Limits for General Dist. System ( 120 V – 69000 V )

Maximum Harmonic Current Distortion in Presence of IL

Individual Harmonic Order ( Odd Harmonics )

ISC/ IL

<20*

20<50

50<100

<11

4.0

7.0

10.0

11≤h<17

2.0

3.5

4.5

17≤h<23

1.5

2.5

4.0

23≤h<25

0.6

1.0

1.5

35≤h

0.3

0.5

0.7

TDD

5.0

8.0

12.0

100<1000 12.0 5.5 5.0 2.0 1.0 15.0

>1000 15.0 7.0 6.0 2.5 1.4 20

Even harmonics are limited to 25% of the odd harmonic limits above.

Current distortion that result in a dc offset, e.g., half-wave converters, are not allowed

*All power generation equipment is limited to these values current distortion, regardless of actual ISC/IL

Filter parameters are designed based on [10, 11]. The maximum of current ripple must be

%10 or %20 of the rated current value by selecting the proper inverter side inductor ( )

which can be derived from the following equation:

(1)

√

√

√

√

Where, is the maximum ripple current,

is the switching frequency,

is the DC bus voltage,

is the rated power,

is the line voltage.

By assuming the maximum ripple current of %20 of the grid peak current, the switching

frequency of 4950Hz, DC bus voltage of 700V, line voltage of 400V, and the rated power

of 100KW, the resulting value of is obtained as 500 µH.


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The filter capacitor value ( ) is usually selected so that the reactive power produced by

it, less than 15% of rated power [10] or 5% of the rated power [11]. If the filter reactive

power is considered as 10% of the rated power, the filter capacitor value will be 100 µF

which is depicted in the following equation:

(2)

√

√

The grid side inductor is selected in a way that the current distortion in the output

should be adjusted to the standard. can be determined as a function of , using the index

for the relation between the two inductors:

(3)

By selecting the desired current ripple attenuation, the value is achieved and then the

value is calculated through equation (3).The ripple attenuation is obtained as [7, 8]:

(4) ( )

( )

| [ ]|

Where ( )

and (

⁄ ) are the switching frequency harmonic

order, ( ) is grid current harmonic at switching frequency, and ( ) is the inverter

current harmonic at the switching frequency.

According to IEEE standard, output current in high order harmonics should be less than

0.003 of the nominal value; therefore, the value of is achieved 0.4 and the value is

calculated to 110 µH.

The resonant frequency of the filter must be between ten times of the grid frequency and

one-half of the switching frequency. It is achieved from equation (5) which in this case it

equals to 1000 Hz:


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(5) √

2.3. Filter transfer function and damping loop

In this section, the dynamic characteristics of the LCL filter are evaluated. Filter transfer

function is indicated in equation (6).For simplicity, resistance elements are not considered.

LCL filter model and transfer function bode diagram are presented in Figure 2 and Figure

3, respectively.

(6)

( ) ( )

1/Lf.s

1/Cf.s

1/Lg.s

ui

ug

uc

if

ic

ig

+-

+-

+-

Figure 2: LCL filter model


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Figure3: LCL filter bode diagram without element's resistance

As it can be seen from the peak resonant filter bode diagram, this system is unstable. For

damping the resonant amplitude, there are many active and passive methods. In passive

method, damping resistance is added to filter circuit. This type of resistance increases loss.

In active methods, capacitor voltage and current are applied and the system is stabilized by

a feedback loop. Damping methods of resonance peak are addressed in [10, 11]. In this

study, according to Figure 4, the feedback is taken from capacitor current. The role of

capacitor current is to reduce the dependence of system behavior to element variation

values and to increase robustness of internal control [10]. In Figure 4, the inverter is

modeled as a gain (M). New system transfer function is represented by:

(7)

( ) ( )

( )

Bode diagram of the system with and without damping loop for different gains are

presented in Figure 5. Moreover, the characteristic of filter without the capacitor is shown


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in Figure 5. By increasing , the system’s transfer function peak in the resonant frequency

is eliminated.

Kc M

PWM

ig

Ic* m ui

uc

ug

1/Cf.s

1/Lf.s

1/Lg.s

if

ic

Figure 4: Control diagram block with damping loop

Figure5: LCL filter bode diagram with/without damping loop


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3. Control Scheme

The proposed control strategy is applied to the grid side converter based on stationary

reference frame which is also known as frame. Since the nonlinear load is connected to

the grid, current harmonic control is simply performed in these axes. As it is represented in

equation (8), the reference current is calculated using instantaneous power theory.

(8) [

]

[

] [

]

The Clarke transformation ( transformation) is necessary to transform the three-phase

instantaneous voltages and currents (feedback variable) into the stationary quadrature

which are given by (9) and (10):

(9) [ ] √

[

√

√

]

[

]

(10) [ ] √

[

√

√

]

[

]

The changes due to variation of active power in DC-side. To control the active

power, DC bus voltage is regulated by the regulator because the variation of

and output power are in the same direction. Therefore, the power flow through the

inverter must be changed. The variation of the is added to the control reference

current as an active power component ( ) according to equation of (11, 12) :

(11)

(12)

(13) ( ) (

)


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Where, , are the proportional and integral gain of the PI controller respectively and

is the instantaneous reactive power. The inverter output current is stabilized and regulated

by PR controller. The block diagram of the internal current controller is shown in Figure 6.

ig

Kc M

PWM

Ic* m ui

uc

ug

1/Cf.s

1/Lf.s

1/Lg.s

if

ic

PRG

Figure 6: Block diagram of the internal current controller

The PR current controller, which is used in this block diagram, is represented by:

(14)

⁄

Where, is the resonant frequency. The parameters of controller are tuned based on the

pole-zero cancellation method.

In the presence of nonlinear loads, the harmonic currents are injected into the grid.

According to the standard, the current harmonic distortion rate should not increase in the

connection point of DG to the grid. The IEEE Standard Table is presented in filter design

section. One of the harmonic current reduction methods are installation of a filter in the

grid. Most of DG units are inverter-based and have filter in the output. Using another filter

in the grid causes various challenges which seem to be hardly overcome. These problems

include the cost rise, the resonance between two filters, and the difficulty in controlling

which is the result of order system increasing.


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Therefore, a solution should be found as a control method to solve unbalanced and

harmonic problems. Since the αβ axes are used, additional components can be compensated

by applying instantaneous power theory and by using an inverter as an active filter. In fact,

the basis underlies the method is adding the unbalanced and harmonic currents created in

PCC to the inverter reference current. The feedback is taken from nonlinear load currents

and PCC voltage and the reference current is created based on which to compensate

nonlinear currents. In this active power compensator, is the oscillatory power and is

the instantaneous reactive power. Harmonic current compensation is indicated in Figure 7

[12].

1 11

2 22

3 3 30

2 2

a

b

c

vv

vv

v

1 11

2 22

3 3 30

2 2

La

Lb

Lc

ii

ii

i

Clarke Transformation

.i .ip v v

. .q v i v i

Instantaneous Power Calculation

Selection of the powers to be compensatedSelection of the powers to be compensated

bvav

cv

Lai

Lbi

Lci

v

v

i

i

p q

-currents calculation

2 2

1 c

b c

i v v p

v vv vi q

cp

cq

i

i

1 0

2 1 3

3 2 2

1 3

2 2

Ca

Cb

Cc

ii

ii

i

Inverse Clarke Transformation

Cai

Cbi

Cci

Figure 7: Harmonic current compensation by using instantaneous power theory [12]


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Figure 8 shows harmonic current flow in the connection point of DG to grid. Nonlinear

load causes 5th, 7

th, 11th, 13

th, and 19

th harmonic currents. Without the DG link control

method, these harmonic currents are drawn from the grid and grid currents are non-

sinusoidal. Therefore, the control method must be designed to provide all harmonic

currents of the nonlinear load and to achieve sinusoidal grid currents with no harmonics.

Figure 8: Harmonic current flow in connection point of DG to grid

To do this, by taking the feedback of nonlinear load currents, active and reactive power

fluctuations are compensated. The feedback is taken from PCC voltage to control voltage

unbalances and to compensate it. Control and feedback signals are shown in Figure 9-a, and

the details of control block depicted in Figure 9-b. As mentioned previously, currents of

filter capacitor are used for active damping and nonlinear load currents are used for

harmonic compensation by the inverter. All the details are presented in Figure 9.


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L

Q1

Q4

Q5

Q2 Q6

Q3

g1 g3 g5

g6

Vdc C

Idc

Idc

Vdc

Lf

Cf

Lg

RL

switch

ILa

ILb

ILc

A

B

C

fault

abc/ab

Va Vb Vc

abc/ab

abc/ab

Ia

Ib

Ic

ILa

ILb

Va

Vb

Ia

Ib

Co

ntro

l Blo

ck

PR

PR

abc/ab

Icc

Icb

Ica

Ib

IaSVM

g1g2g3

g5g6

g4

P*

P*Q*

I*a

I*b

Rs Ls ACVa

Vb

Vc

g2 g4

Figure 9-a: Block diagram of system and control strategy

700

Vdc

PI

P*

Va

ILa

VbILb

mean

Pref

P_oscilation

Po

Va

ILb

VbILa

Q* Qo

Va

Vb

(Va^2+Vb^2)

Iac

(Va^2+Vb^2)

Vb

Va

Ibc

Iac

Ibc

Ia

Ib

I*a

I*b

Figure 9-b: Details of control block in Figure 9-a


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4. Dynamic Model of DG System

In the previous section, the power supply is assumed as a controlled current source and

the control circuit is designed according to that. In this section, the DG source is modeled

by a wind turbine as it is shown in Figure 10. Thus, the internal dynamics and their effects

on the control circuit are evaluated [13].

Figure 10: Wind turbine model

Its CP - λ characteristics, as Figure 11 depicts, are used to achieve maximum active

power.


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Figure11: Cp – λ curve for wind turbine

In addition, a permanent magnet synchronous generator (PMSG) in torque control mode

is considered as a wind turbine generator (Figure 12).

Figure12: Wind turbine generator model


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5. Simulation

To describe and validate the proposed control method, simulations are carried out for

a distribution grid connected to a DG link in presence of nonlinear load and different

types of the faults in Matlab/Simulink environment. The DG source is considered as

100KW controlled current source and 100KW PMSG-based wind power generation

system separately. The inverter output voltage is 400V. The 30KW nonlinear load is a

full-wave diode converter supplies a resistive load in each phase which generates 6k±1

order harmonic currents. Results are given in the remaining of this section.

5.1. Simulation in normal condition

In normal condition, the three-phase grid currents are shown in Figure 13.

Figure13: Grid currents in normal condition

The Power Factor (PF) which is equal to 1 is shown in Figure 14.


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Figure 14: Grid PF waveform

The grid current harmonic spectrum is shown in Figure 15.

Figure 15: Grid current harmonic spectrum

In this part, a 50KVAR reactive power is injected to the grid at . The grid

voltage and the current in phase "a" are shown in Figure 16 and the grid power factor is

presented in Figure 17. As can be seen, the current is lagging the voltage.


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Figure 16: Grid voltage and current in phase "a", when inverter injects 50 KVAR to grid.

Figure 17: Grid power factor

50KVAR is absorbed by the inverter at . The grid voltage and grid current in

phase "a", and the power factor are shown in Figure 18 and Figure 19, respectively. As can

be seen, the current is leading the voltage.


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Figure 18: Grid voltage and current when inverter absorbs 50KVAR reactive power

Figure 19: Grid Power Factor

5.2. Simulation in presence of nonlinear load

In the next stage, the nonlinear load is connected to the PCC at . Before the

connection of DG link control circuit to the grid, the AC grid needs to provide harmonic

currents for the nonlinear load. After the connection of DG link to the distribution grid

at , the load harmonic currents are fed from it and the grid currents are

sinusoidal .Figure 20 shows the changes of grid currents during the connection of DG

source to the distribution grid .The load currents are shown in Figure 21.


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Figure 20: grid currents before and after the connection of DG to grid in presence of nonlinear load

Figure 21: nonlinear load currents

In addition, the THD of the grid current is shown in Figure 22. As it can be observed,

after the connection of DG source to the grid, the THD value of grid current is decreased

and it is placed in the standard level. The harmonic currents of the load, the grid, and the

inverter before and after the connection of DG link are presented in Figure 23 to evaluate

the capabilities of DG link control circuit in order to compensate the load harmonic

currents.


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Figure 22: THD of grid current

Before the connection of the DG source to the grid, nonlinear load draws harmonic

current from the grid. After the connection of DG link control circuit to the distribution

grid, it provides all harmonic currents of the loads and the harmonic currents which are

dawn from the grid decreased to zero. It should be noted that the currents which are shown

at Figure 23 are rms values of 5th

, 7th

, 11th, 13th

, and 19th

harmonic currents.

Figure 23: Harmonic currents of inverter, nonlinear load and grid


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5.3. Simulation in the presence of unbalanced voltage and fault

To evaluate the appropriate performance of the proposed control method, it is

examined against different types of the faults. In this case, the three-phase to ground

fault is occurred at and it is removed at .The grid voltages are

shown in Figure 24. Moreover, the grid voltage and current in phase “a” and the DC

bus voltage are shown in Figure 25 and 26, respectively.

Figure 24: Grid voltages during occurrence of three-phase fault

Figure 25: Grid voltage and current in phase “a”

Figure 26: DC bus voltage


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It is observed from the above Figures that, after the fault is cleared, voltages and

currents of the grid are recovered to initial values and the grid is able to continue normal

operation. Then, a double phase to ground fault is occurred in phases “a” and “b” from

to . The grid behavior is evaluated without the local load. As it can be seen

in Figure 27, after the fault is removed, the grid reaches its normal stability and it returns to

its initial state. Figure 26 shows that DC bus voltage remains stable during the fault.

Figure 27: Grid voltages when two-phase fault is occurred

At the end of this section, the grid is evaluated in presence of 30KW nonlinear load and

the double phase to ground fault that is occurred between and .

The grid voltages and the DC bus voltage are shown in Figures 28 and Figure 29.As it can

be seen in Figure 28, the system's stability can be guaranteed under this condition as well.

Figure 29 shows that DC bus voltage does not exceed significantly the rated value.


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Figure 28: Grid voltages

Figure 29:Dc bus voltage when double phase fault to ground is occurred

5.4. Simulation in the presence of wind turbine

In this section, DG dynamics are applied in the simulation and performance of control

circuit is evaluated. Mechanical and electrical power of wind turbine is illustrated in Figure

30.


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Figure 30: Input and output powers of wind turbine

5.4.1. Without nonlinear load: In this section, simulations are performed without the

nonlinear load. The DC bus voltage is shown in Figure 31.

Figure 31: DC bus voltage

Figure 32: Grid current in starting mode of turbine and without nonlinear load


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Figure 33: Grid current THD in starting mode of turbine and without nonlinear load

Figure 34 shows the grid voltage and current in phase “a”, when the inverter injects

50KVAR to the distribution grid at t . As can be seen, the current is lagging the

voltage.

Figure 34: Grid voltage and current in Phase “a”

5.4.2. In the presence of nonlinear load: The nonlinear load is connected to the grid at

.The wind energy source is connected to the grid using the mentioned

control method at . The grid currents and the harmonic currents of the load, the

grid, and the inverter are shown in Figures 35 and 37 respectively. It is obvious that

after the connection of wind turbine control circuit to the grid, it provides all harmonic

currents of the loads and AC grid does not need to provide them for the nonlinear

loads. As it can be seen in figure 36, after the connection of wind turbine control

circuit to the grid, the THD of the grid current is decreased.


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Figure 35: Grid currents before and after the connection of wind energy source to grid in presence of

nonlinear load

Figure 36: Grid current THD

Figure 37: Harmonic currents of inverter, nonlinear load and grid


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Conclusion

A flexible control method to connect DG resources to the distribution grid has been

proposed in this paper. The proposed control strategy has shown benefits for robust control

against harmonic distortion in DG applications. Simulation results have illustrated that in

presence of nonlinear load, AC grid does not need to provide harmonic currents for the

nonlinear loads and the proposed DG system can provide required harmonic load currents

in all conditions. Therefore, by reducing THD of grid current it can act as an active filter.

Simulation results have indicated that, the system's stability can be guaranteed under

unbalanced voltage condition. Other advantages of the proposed control method include the

compensation of the reactive power variation and the correction of the power factor. The

proposed control method can be used for different types of DG resources as power quality

improvement devices in the distribution grid.

References

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connected inverters for power quality enhancement: A comprehensive review", Renewable and Sustainable

Energy Reviews, Vol. 24, (2013), pp. 223-270.

[2] C.L.T. Borges and M. Falcão Djalma, "Optimal distributed generation allocation for reliability, losses, and

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[3] J.M. Carrasco, L.G. Franquelo, J.T. Bialasiewicz, E. Galvan, R.C.P. Guisado, M. Prats, J.I. Leon and N.

Moreno-Alfonso, "Power-Electronic Systems for the Grid Integration of Renewable Energy Sources: A

Survey", IEEE Transactions on Industrial Electronics, Vol. 53, (2006), pp. 1002-1016.

[4] M.H. Bollen, "Understanding Power Quality Problems: Voltage Sags and Interruptions", IEEE Press: New

York, (1999).

[5] E. Twinning and D.G. Holmes, "Grid current regulation of a three-phase voltage source inverter with an

LCL input filter", IEEE Transactions on Power Electronics, Vol. 18, (2003), pp. 888-895.

[6] Q. Zeng and L. Chang, "Development of an SVPWM-based predictive current controller for three-phase

grid-connected VSI", Fortieth Industry Applications Conference Annual Meeting, (2005) October p.2395- 2400.


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[7] M.P. Kazmierkowski and L. Malesani, "Current control techniques for three-phase voltage-source PWM

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Authors

Hamed Tahami received his B.Sc. degree in electrical power engineering from Babol

Noshirvani University of technology in 2010. Then, he attended in the same university

to get his M.Sc. degree in electrical power engineering. His research interests include

power electronics application in power system, distributed generation system and its

control methods, and the integration of renewable energy resources into the power

grid.

Seyed Mehdi Hosseini was born in Sari, Iran, on September 18, 1978. He received the

B.Sc. and M.Sc. degrees in electrical engineering in 2000 and 2002 and the PhD

degree in electrical engineering from Iran University of Science & Technology

University (IUST) in 2009. Presently, he is assistant professor at electrical engineering

department of Babol Noshirvani University of Technology, Babol, Iran. His research

interests are the application of Flexible AC Transmission Systems (FACTS) devices in

transmission systems, application of Distribution FACTS (D-FACTS) devices in

distribution systems, distributed generation and reliability of distribution systems.

Jafar Adabi was born in 1981. He received his B.Sc. and M.Sc. degrees from

Mazandaran University, Babol, Iran, in 2004 and 2006, respectively. He finished his

PhD degree in the School of Engineering Systems at the Queensland University of

Technology, Brisbane, Queensland, Australia at 2010. Currently, he is an assistant

professor at Noshirvani University of Technology, Babol, Iran. His research interests

are renewable energy systems, the optimal design and high frequency modeling of

power electronics and motor drive systems for EMI analysis.

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