power flow control of micro grid in islanded mode operation

8/20/2019 Power Flow Control of Micro Grid in Islanded Mode Operation

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IPASJ International Journal of Electrical Engineering (IIJEE) Web Site: http://www.ipasj.org/IIJEE/IIJEE.htm

A Publisher for Research Motivation........ Email: [email protected]

Volume 3, Issue 10, October 2015 ISSN 2321-600X

Volume 3, Issue 10, October 2015 Page 1

ABSTRACT

This paper proposes the control strategy to improve the system operation performance. The proposed micro grid consists of a

photovoltaic array, proton exchange membrane fuel cell and lithium ion storage battery. The battery is included in the micro

grid to reduce the burden of the power generated by the micro grid during the peak period. An overall energy management

system is also implemented for the micro grid to coordinate load sharing among different DG units during both grid-connected

and islanded operation. The total system improves the controlling of power Quality and Reliability of the system that the micro

grid is connected to. Kalman filters used to extract the harmonic component of the distorted source voltage and load current,

and estimate the gain and THD. The simulation results shows that the functions of the Distributed Generation Units in the

Micro Grid can be in corporate efficiently under the proposed controller to ensure the stable operat ion of the overall grid.

Keywords: Distributed Generation(DG) inverters, Micro Grid, Kalman Filters, Output Regulation Control,UPQC.

1. INTRODUCTION

Distributed generation (DG) has been reincarnate after its demise by centralized generation. However, DG integrationinto distribution networks leads to a number of challenges. The most interesting micro grid's applications currently arewind power and solar applications So this research limits itself to energy from sun and wind without being unmindfulof other DG technologies. Micro grids are systems which operate with different types of loads and micro sources. Dueto high penetration of distributed generation (DG) units with different types of loads can cause power quality and powercontrol issues An example of the micro grid is shown in Fig. 1, grid connected and islanding being the two operationmodes of this system. In grid-connected mode, the grid dominates most of the system dynamics and no significantissues need to be addressed except the power flow control, whereas in the islanding mode, once the isolating switchdisconnects the utility due to network fault or market decision, the micro sources directly influence the systemdynamics, thus a reliable operation can be only achieved by adopting an adequate control mode.

Figure 1:An Example of Micro Grid

The integration of DG units in the distribution grid will lead to a more complex flow of real and reactive power, whichcan change the voltage profile at the load side during normal operations, and a more complex flow of fault currents for

any fault occurring in the micro grid. A voltage and frequency droop control methods are used for sharing active andreactive power from multiple distributed generation units The total load demand shared by multiple DG units withdifferent conventional droop control. The power sharing at the steady state is always accurate while reactive powersharing is sensitive to the impacts of mismatched feeder impedance. For a networked micro grid configuration with

Power Flow Control of Micro Grid in Islanded

Mode OperationA.Ramesh

1, P.Sravan Kumar

2, S.V.D.Anil Kumar

3

1A.Ramesh, PG Scholar(Power Systems), Department of EEE, St.Ann’s College Of Engineering &Technology ,Chirala,Andhra Pradesh ,India,

2P.Sravan Kumar, Asst. Professor, Department of EEE ,St. Ann’s College Of Engineering &Technology ,Chirala, AndhraPradesh,India,

3S.V.D.Anil Kumar, Assoc. Professor & HOD, Department of EEE , St.Ann’s College Of Engineering &Technology,Chirala,Andhra Pradesh, India,


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linear and non linear loads, the reactive power sharing is more challenging. For improvement of reactive power sharingand control in networked micro grid, a control method is introduced to reduce reactive power sharing errors byinjecting a small real power disturbance.

This paper represents the micro source devices are connected to the utility grid through dc to dc converters and voltagesource inverters at the point of common coupling (PCC).Tests will be conducted using the proposed micro grid and itscontrol system on the following technical issues:1) Power quality improvement in the distribution grid;

2) Real and reactive power management during grid-connectedand islanded operations;3) Network voltage changesAs such modern control theories are used to implement the system. Such controllers’ presents in this paper as outputregulation based controller (ORC).This controller regulates the dc link voltage of the system

2.SYSTEM DESCRIPTION

The figure2 shows the configuration of micro grid presented in this paper that is operated to operate grid connected and

islanded mode of operation. The proposed micro grid consists of 15KVA micro turbine (MT), 5KW photovoltaic array.The PV array is connected to grid through dc to dc converter and voltage source inverter (VSI). The micro grid and pvarray operates as primary generation units. 20Ah of lithium ion storage battery (SB) and 5KW proton exchangemembrane fuel cell (PEMFC). The SB is connected to grid through buck boost converter and VSI. The PEMFC and SBare back up protection of the PV array and will supply. The power for any shortage in the generated power to ensure thestable operation of the overall system. When the fault occurs on the upstream of the distribution network the circuit breaker is disconnect the micro grid from utility grid Then the generation of the main DGs units are able to meet theload demand the system must be stable otherwise PEMFC and SB are supply the power unless it cannot meet the loaddemand shutdown the system completely.The dc-link voltages of the DG inverters are regulated by their respective rectifier, dc/dc boost converter, and bidirectional dc/dc buck-boost converter at a desired voltage as seen in Fig. 2 and the respective DG unit will deliverthe necessary power to maintain the power balance in the dc-link. During grid-connected operation, the power balanceof the overall distribution system is achieved through the distribution grid. However, when the micro grid transits tooperate islanded from the distribution grid, the SB and the PEMFC stack can maintain the power balance in thedistribution system according to the following power balance equation

Pmt+Pav+P b+Pfc=PL --------------- (1)Here Pmt and Ppv are power delivered by the MT and PV array. And P b is the storage battery power subjected tocharging and discharging constraints is shown in the figure 2.

P b≤ P b,max --------------- (2)

Figure 2: Overall proposed micro grid system


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Figure 3: Operation of the SB during grid‐connected operation

Pfc is the power delivered by the PEMFC and PL is the load power .The storage battery energy constraints is given by

SOCmin < SOC ≤ SOCmax -----------(3)

SOC of the battery cannot be determined directly, it can be find by different techniques is presented.

Figure 4: Operation of the SB and PEMFC stack during islanded operation

2.1 DG Inverter Modeling:

The figure shows the single phase circuit representation of DG inverters during grid connected and islanded operationrespectively. The output voltage across the DG inverters is U jVdcj Where U j is the control input i.e j=1,2. And Lf ,Cf isthe filter which protects from the high switching frequency harmonic. R f is the resistance loss of the DG inverter. Theload current iL is the sum of current delivered to the load. iL includes the fundamental and harmonic componenttherefore iL can be written as

iL =iL1+iL2 =iLf +iLh =iLf sin(wt-θLf )+Σ

Nh=3,5.iLhsin(hwt+θLh)

=iLf,p+iLf,q +iLh ------------- (4)

Where θLf and θLh are the respective phase angles of the fundamental and harmonic component of the load currentiL..The current supplied by the DG inverter is given by

IDGj=(iLf,p-ig)+iLf,q +iLh ------------ (5)


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Figure5: Equivalent circuit of DG inverter of during grid-connected

Figure 6: Equivalent circuit of DG inverter of during Islanded connected

Where ig is the grid current. With the power electronics devices, the micro grid current i mg and grid voltage Vg could bedistorted. The grid voltage is also fundamental and harmonic components can be written as

Vg=Vf +Vh =Vf sinwt+ Σ Nh=3,5 Vh sin(hwt-θh) -------------------- (6)

Where θh is the harmonic component of the grid voltage. For compensate the harmonic in V g the series voltage sourceinverter injects the voltage is given by

Vinj=Vh –Vz – Vt -------- (7)

Vt and Vz is the voltage drop across the line impedance and voltage drop across the equivalent leakage reactance of theseries connected transformer respectively. To derive the state space model of the DG inverter during grid connected andislanded operations, is to simply apply the Kirchhoff’s voltage and current law to the network is shown in the figure 7.

i j- VDGJ+ U j -------(8)

i j- iDGj ------------- (9)

Figure 7: Single--‐ phase representation of the DG inverter during both grid--‐connected and islanded operations


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Where i j is the current flowing through Lfj.therefore grid connected DG inverter can be modeled in state space can bewritten as

Ẋ=Agjxgj+Bgj1 v j+Bgj2u j ------------- (9)

ygj=Cgjxgj+Dgj1v j+Dgj2uj --------- (10)

Where g represents the grid connected operation and j represents the operation of the DG inverter( j=1,2)(1=MT,2=PV).

Agj= ;Bgj1= ; Bgj2= ;

Cgj=1;Dgj1=[0 -Cfj]];Dgj2=0

xgj=i j is the state v j=[vdgj ]T is the exogenous input,u j is the control input, with -1≤uj ≤1; and ygj= iDG j is the output.

The islanded operation of SB and PEMFC is derived in state space model is obtained

Ẋij=Aijxij+Bgj1 i j+Bij2u j -------------- (11)yij=Cijxij+Dgj1i j+Dij2u j ---------- (12)

Where i denotes model of the DG inverters j during islanded operation. where j=1,2,3,4 and

Aij= ;Bij1= ;Bij2= ; Cij= ;Dij1= ;Dij2=

With Cf r =∑4 j=1Cfj ; xij= [ i j VDGj ]T T is the stete vector; I

r j =iL-∑n≠j in is the exogenous input of the DG inverter j; u j is

the control input , with -1≤ u j ≤1 and yij= [VDGj iDGj ]T is the output, which will regulated to track the desiredreference. can be derived using a Kalman filter applied to the output yij= [VDGj iDGj ]T.

3. Unified Power Quality Conditioner TopologyThe Unified Power Quality Conditioner is a custom power device that is employed in the distribution system to mitigatethe disturbances that affect the performance of sensitive and/or critical load.

Figure 8: Functional Block Diagram of UPQC Controller:

The main components of a UPQC are series and shunt power converters, DC link, low-pass passive filter and high-pass passive filters, and series transformer and shunt transformers.


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Figure 9: Equivalent single-phase representation of the UPQC:

Figure 10: The equivalent single-phase representation of the UPQC in a power distribution system .

4 CONTROL METHODOLOGYThe aim of the controller is to maintain the output of the plant is y=(vmg ig)

T it is to track the reference signal r=( vmg *ig*)T , under a periodic disturbance (vmg ig)T . The reference signals vmg* and ig * both are pure sine waves 50Hzfrequency without any harmonic distortions.4.1

Model of the exogenous signal

The exogenous signals vg ,iL ,vmg * ,ig * are periodic signals all those are represented in state space model. For examplevg can be expressed in state space model is

ξvg =Avg ξ vg ------------ (12)vg=C vg ξ vg ------------ (13)

iL ,vmg * ,ig* are modeled into state space form

ξ = Ã ξ T --------------- (14)

(vg iL)T =Ĉw ξ +w -----------(15)

r=(vmg* ig*)T= Ĉd ξ ----------- (16)

Which is called as exogenous system is presented in this paper.

4.2 Output based regulation controller (ORC)

Subtitling the exogenous equations into the state space model of the plant, the following model is obtained

Ẋ=Ax+B0 ξ

+B2u -------------- (17)y=Cx+D0 ξ +D2u ------------- (18)


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the control law for the OR is proposed asu=U ξ +F(x-X ξ) ----------(19)

Where u is the control signal which generated by the controller used for switching scheme for the plant. x and ξ are the outputs of the exogenous and plant kalman filter.

XÃ=AX+BO+B2U ------------ (20)

CX+DO+D2U ---------- (21)

(x-X ξ)=(A+B2F)(x-X ξ)-------- (22)

e=(C+D2F)(x-X ξ) -----------23)

Where ‘e’ is the error tracking (y-r). F can be find from the (A+B2F) is Hurwitz and the closed loop system is stable.themodel of ORC is shown in figure below.C.Kalman Filters

The existence of harmonics in any power system is increasingly drawing attention due to wide applications of powerelectronic devices. These harmonics are not desirable because they degrade power quality and lead to stability problems. Kalman Filter model shows some limitations in noisy environment. In this paper a model of Kalman Filter with ORC is used for harmonic estimation in the micro grid.

Figure 11:The Model Of Out put Regulation Controller

V. SIMULATION RESULTS

The simulation model of the micro grid is implemented in MATLAB/SIMULINK is shown in the figure. The microgrid consists of two types of loads i.e non liner and linear loads rated of P L1= 15KW, Q L1=9.7KVAr and PL2=5KW, QL2=3KVAr are respectively.

Assessment 1: Power Quality Improvement With Load Sharing During Grid-Connected Operation

This assessment demonstrates that the improvement of power quality of the distribution network during grid connectedoperation and mitigation of harmonic injection in the distribution network during grid connected operation. In thismode of operation the storage battery is charging and stores the energy for 0≤t≤0 .4sec.The total load current is thesummation of the micro grid current and current supplied by the grid current.During steady-state condition, the total harmonic distortion (THD) value of Grid Voltage is as shown in Figure 14.With the main DG unit compensating for the harmonic contents as shown in Figure 10 the THD value of Vg isimproved to about 0.43% as shown in Figure 14. To achieve power factor correction at the grid side, the main DG unitis also controlled to provide the reactive component V Lf,q of the current Vg is as shown in figure 13.The waveform of load current iL1 (top), current supplied by the DG1(middle) and grid current (bottom) is shown in thefigure. The below figure shows the grid voltage and current wave form.

The total real and reactive power delivered by the load is 20KW and 12.7KVAr during connected mode of operationthe DG1 inverter and DG2 inverter shares the power is DG1=7.5KW and DG2 is 3KW is shown in the figure. Reactive power supplied the load is shares the DG1 and DG2


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0.1 0.15 0.2 0.25 0.3 0.35 0.4

-50

0

50

TIME(SEC)

L O A D 1

C U R R E N T ( A )

0.1 0.15 0.2 0.25 0.3 0.35 0.4-100

-50

0

50

100

TIME(SEC)

D G 1

C U R R E N T ( A )

0.1 0.15 0.2 0.25 0.3 0.35 0.4-50

0

50

TIME(SEC)

G R I D

C U R R E N T ( A )

Figure 12: load1current (top),DG1current (middle),gridcurrent(bottom)

0.1 0.15 0.2 0.25 0.3 0.35 0.4-80

-60

-40

-20

0

20

40

60

80

TIME(SEC)

V O L T A G E ( V )

C U R R E N T ( A )

VOLTAGE(V)

CURRENT(A)

Figure:13 voltage and current waveform of grid

Figure:14 FFT analysis of Grid Voltage


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0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

x 104

0

5000

10000

15000

TIME(SEC)

R E A L P O W E R ( K W )

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4-3

-2

-1

0

1

2

3x 10

4

TIME(SEC)

R E A C T I V E P O W E R ( K V A R )

Figure:15: Power Supplied by MTG

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.40

2000

4000

6000

8000

10000

12000

14000

R E A L P O W E R ( K W )

Figure:16 Power Supplid by PV array


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The residual power is supplied by the grid is shown in thefigure.13. 7.5KW (50% of PL1) and 2KW(20% of PL2).

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.40

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

TIME(SEC)

R E A L P O W E R ( W A T T S )

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

TIME(SEC)

R E A C T I V E P O W E R ( K V A R )

REACTIVE POWER DELIVERED BY THE GRID

Figure 17: Power Supplid by Grid

Assessment 2: load sharing during Islanded operation:

This assessment verifies the islanded mode operation of micro grid. The circuit breaker is primarily fully cut off, whenever fault occurs on the main grid network, the circuit breaker disconnects the micro grid from the grid. This test isemployed to for a moment to provide the sharing of real and reactive power of storage battery SB and PEMFC. Duringthe interval of 0≤t≤0.4sec, at 0.2sec the circuit breaker switch opened, the amount of real power delivered by the SBand PEMFC is shown in the figure.16.at t=0.2 sec the SB is supplies the 3.28KW and PEMFC is supplies the2.28KW.Iinitially the SB is idle or charge now in islanded mode the SB is supplies the power is shown in the figure.17.The real power is supplied by the MT and PV array is increased

Figure 18:FFT analysis of MT Voltage In Islanded Mode


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During Islanded mode condition, the total harmonic distortion (THD) value of MT Voltage is as shown in Figure 18.With the main DG unit compensating for the harmonic contents,the THD value of Vmt is improved to about 0.10% as

shown in Figure 18.

0.1 0.15 0.2 0.250

2000

4000

6000

8000

10000

12000

14000

TIME(SEC)

M T P O W E R ( W )

POWER DELIVERED BY MICRO TURBINE

Figure 19: Power delivered by MTG

0.1 0.15 0.2 0.250

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

TIME(SEC)

P O W

E R ( W )

POWER DELIVERED BY PV ARRAY

Figure 20: Power delivered by PV array

0.1 0.15 0.2 0.250

500

1000

1500

2000

2500

3000

3500

4000

TIME(SEC)

P O W E R ( W )

POWER DELIVERED BY BATTERY

Figure 21: Power delivered by battery


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0.1 0.15 0.2 0.250

500

1000

1500

2000

2500

3000

3500

4000

TIME(SEC)

P O W E R ( W )

POWER DELIVERED BY FUEL CELL

Figure 22: Power deliverd by PEMFC

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-500

0

500

TIME(SEC)

V O L T A G E ( V O L T S )

VOLTAGE AND CURRENT WAVEFORM OF LINEAR LOAD

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-10

-5

0

5

10

TIME(SEC)

C U R R E N T ( A M P )

Figure 23:Voltage and Current waveforms of Linear Load

Figure 24:Voltage and Current waveforms of Non-Linear Load


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VI. CONCLUSION

This paper presents the functioning of micro grid during both grid connected and islanded mode of operation and has been presented with the help of MATLAB/SIMULINK. The considered system incorporates with kalman filter in the

control design and is to observe the gain and extracts the harmonic contents in the supply voltage and load current.The results obtained validate that the micro grid can handle different loading conditions effectively during gridconnected and islanded mode of operations. From these test results the proposed micro grid system improves the powerquality and stability.

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power flow control of micro grid in islanded mode operation

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