Download International Electrical Engineering Journal (IEEJ) Vol. 5 (2014) No.10, pp. 1553-1558

International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.10, pp. 1553-1558
ISSN 2078-2365
http://www.ieejournal.com/
Grid and Islanded Operation of a
Distribution Generation Inverters in a
Microgrid
N.Saida Naik, J.Bhavani
[email protected], [email protected]
Abstract— This paper presents the design of a micro grid. The
proposed micro grid consists of a photo voltaic array which
represents the main generation unit in the microgrid and
proton exchange membrane fuel cell is supplement the variable
power generated by the photovoltaic array. a lithium ion
battery is included in the microgrid for reduce the burden of
the power generated by the microgrid during the peak period.
The all those different dg’s units is coordinate to operate the
energy management systems during the grid connected
operation. The overall system improves the power quality and
reliability of the power distribution system that the microgrid
is connected to. The control design employs the output
regulation (OR) theory. Kalman filters used to extract the
harmonic component of the distorted source voltage and load
current, and estimate the state observer gain and frequency
tracking .The simulation studies verified through different test
case.
Index Terms— Distributed generation (DG), microgrid,
output based controller (ORC), kalman filter.
I. INTRODUCTION
Now a days using of the power electronics devices poses a
power quality problems and the concept of microgrid is
useful (helpful) to consumer for reliable and efficient power
supply and reduce [1] the per unit cost of the electrical load
and reduce the total energy losses and also the
interconnection of microgrid to the utility grid with the help
of power electronics devices poses power quality problems.
The power quality problems like voltage sag/swell,
frequency deviation, harmonic supply voltage and load
current in the microgrid.
To overcome all those power quality problems uses
different power conditioning devices. Such as like active
power filters [2], uninterruptible power supplies [3],
dynamic voltage restorers [4] and unified power quality
conditioners [5] are protected their consumers loads and
protects the system against the power quality problems,
This paper represents the microsource devices are
connected to the utility grid through dc to dc converters and
voltage source inverters at the point of common coupling
(PCC). At point of common coupling different electrical
Loads are connected to. The proposed microgrid consists of
a photovoltaic array, proton exchange membrane fuel cell
and lithium ion storage battery. The photovoltaic array
operates main generation unit of the microgrid. During the
sunless hours proton exchange membrane fuel cell operates
the main generation unit. The lithium ion storage battery is
implemented to operate the peak shaving during the grid
connected operation and islanded operation. As such
modern control theories are used to implement the system.
Such controllers’ presents in this paper as output regulation
based controller (ORC) [6-7], this controller regulates the dc
link voltage of the system.
II. SYSTEM DESCRIPTION AND MODELLING
The figure1 shows the configuration of microgrid presented
in this paper that is operated to operate grid connected and
islanded mode of operation. The proposed microgrid
consists of 15KVA micro turbine (MT), 5KW_p
photovoltaic array. The PV array is connected to grid
through dc to dc converter and voltage source inverter
(VSI). The microgrid and pv array operates as primary
generation units. .20Ah of lithium ion storage battery (SB)
and 5KW proton exchange membrane fuel cell (PEMFC).
The SB is connected to grid through buck boost converter
and VSI. The PEMFC and SB are back up protection of the
PV array and will supply the power for any shortage in the
generated power to ensure the stable operation of the overall
system.
When the fault occurs on the upstream of the distribution
network the circuit breaker is disconnect the microgrid from
utility grid .Then the generation of the main DGs units are
able to meet the load demand the system must be stable
otherwise PEMFC and SB are supply the power unless it
1553
Saida and Bhavani
Grid and Islanded Operation of a Distribution Generation Inverters in a Microgrid
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.10, pp. 1553-1558
ISSN 2078-2365
http://www.ieejournal.com/
cannot meet the load demand shutdown the system
completely.
Figure3: Operation of the SB and PEMFC stack during islanded operation A. DG inverter modeling (12)
Figure1: Overall proposed microgrid system
Power balance equation of the system during grid connected
mode is
Pmt+Ppv+Pb+Pfc=PL
(1)
Here Pmt and Ppv are power delivered by the MT and PV
array. And Pb is the storage battery power subjected to
charging and discharging constraints is shown in the figure
[2-3].
Pb ≤ Pb,max
(2)
The figure [4-5] shows the single phase circuit
representation of DG inverters during grid connected and
islanded operation[10],[14] respectively. The output voltage
across the DG inverters is UjVdcj. Where Uj is the control
input i.e j=1,2. And Lf,Cf is the filter which protects from
the high switching frequency harmonic. Rf is the resistance
loss of the DG inverter. The load current iL is the sum of
current delivered to the load. iL includes the fundamental and
harmonic component therefore iL can be written as
iL =iL1+iL2 =iLf+iLh
=iLfsin(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 current
iL..The current supplied by the DG inverter is given by
IDGj=(iLf,p-ig)+iLf,q+iLh
(5)
Figure2: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
Figure4: Equivalent circuit of DG inverter of during grid-­‐connected (3)
SOC[8-9] of the battery cannot be determined directly, it
can be find by different techniques is presented.
1554
Saida and Bhavani
Grid and Islanded Operation of a Distribution Generation Inverters in a Microgrid
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.10, pp. 1553-1558
ISSN 2078-2365
http://www.ieejournal.com/
Figure 5: Equivalent circuit of DG inverter of during Islanded connected Where ig is the grid current. With the power electronics
devices, the microgrid current img and grid voltage Vg could
be distorted. The grid voltage is also fundamental and
harmonic components can be written as
Vg=Vf+Vh
= Vfsinwt+Σ Nh=3,5.Vhsin(hwt-θh)
xgj=ij is the state Vj’=
Vinj=Vh –Vz – Vt
(11)
=
vDGJ+
i j-
iDGj
(10)
Where i denotes model of the DG inverters j during islanded
operation.where j=1,2,3,4 and
Aij=
i j-
=Aijxij+Bij1 ij’+Bij2uj
yij=Cijxij+Dij1ij’+Dij2uj
(7)
Vt and Vz is the voltage drop across the line impedance and
voltage drop across the equivalent leakage reactance of the
series connected transformer respectively.
To derive the state space model of the DG inverter during
grid connected and islanded operations, is to simply apply
the Kirchhoff’s voltage and current law to the network is
shown in the figure6.
=
is the exogenous input,
uj is the control input, with -1≤uj ≤1; and ygj= iDGj is the
output.
The islanded operation of SB and PEMFC is derived in state
space model is obtained
(6)
Where θh is the harmonic component of the grid voltage. For
compensate the harmonic in Vg the series voltage source
inverter injects the voltage is given by
T
uj
;
Bij1=
Dij1=
Bij2=
; Cij=
; Dij2=
T
=
;xij=
is the stete vector; =iLis the exogenous input of the DG inverter j; uj is the
T
control input , with -1≤uj ≤1 and yij=
is the
output, which will regulated to track the desired reference.
can be derived using a Kalman filter applied to the output
T
yij=
.
With
III. CONTROL PHILOSOPHY
The 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 50Hz
frequency without any harmonic distortions.
a. Model of the exogenous signal
Figure6: Single-­‐phase representation of the DG inverter during both grid-­‐connected and islanded operations Where ij is the current flowing through Lfj.therefore grid
connected DG inverter can be modeled in state space can be
written as
=Agjxgj+Bgj1 vj’+Bgj2uj
ygj=Cgjxgj+Dgj1vj’+Dgj2uj
(8)
(9)
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=
Dgj1=
Bgj2=
; Dgj2=0
; Cgj=1
The exogenous signals vg ,iL ,vmg* ,ig* are periodic signals
all those are represented in state space model. For example
vg can be expressed in state space model is
ξVg=AVg ξVg
vg=CVg ξVg
(12)
(13)
iL ,vmg* ,ig* are modeled into state space form
=Ãξ
(vg iL)T =Ĉw ξ +w
(14)
(15)
r=(vmg* ig*)T= Ĉd ξ
(16)
Which is called as exogenous system is presented in this
paper.
1555
Saida and Bhavani
Grid and Islanded Operation of a Distribution Generation Inverters in a Microgrid
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.10, pp. 1553-1558
ISSN 2078-2365
http://www.ieejournal.com/
b. Output based regulation controller (ORC)
Subtitling the exogenous equations into the state space
model of the plant, the following model is obtained
= Ax+ B0ξ + B2u
y= Cx + D0 ξ +D2u
(17)
(18)
the control law for the OR is proposed as
(19)
d
( x − X ξ ) =(A+B2F)(x-X ξ)
dt
LOAD1
CURRENT(A)
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(13).
XÃ=AX+Bo+B2U
(20)
=CX+Do+D2U
(21)
50
0
-50
0.1
0.15
0.2
0.15
0.2
0.25
TIME(SEC)
0.3
0.35
0.4
0.25
0.3
0.35
0.4
0.3
0.35
0.4
100
DG 1
CURRENT(A)
u = U ξ + F(x - X ξ )
50
0
-50
-100
0.1
TIME(SEC)
50
(22)
e=(C+D2F)( x-X ξ)
(23)
GRID
CURRENT(A)
:
the DG1 (middle) and grid current (bottom) is shown in the
figure.8. the below figure.9 shows the grid voltage and
current waveform .
The total real and reactive power delivered by the load is
20KW and 12.7KVAr during grid connected mode of
operation the DG1 inverter and DG2 inverter shares the
power is DG1=7.5KW and DG2 is 3KW is shown in the
figure.10, reactive power supplied the load is shares the
DG1 and DG2
0
-50
0.1
0.15
0.2
0.25
TIME(SEC)
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.the
model of ORC is shown in figure below.
Fig.8:load1current (top),DG1current (middle),gridcurrent(bottom)
80
VOLTAGE(V)
CURRENT(A)
60
VOLTAGE(V)
CURRENT(A)
40
20
0
-20
-40
-60
-80
0.1
0.15
0.2
0.25
TIME(SEC)
0.3
0.35
0.4
Fig9. voltage and current waveform of grid
15000
IV. SIMULATION RESULTS
10000
REAL POWER(KW)
The simulation model of the microgrid 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 PL1= 15KW, QL1=9.7KVAr and
PL2=5KW, QL2=3KVAr are respectively.
5000
0
0
Test case1: Power Quality Improvement With Load Sharing
During Grid-Connected Operation
x 10
1
1.5
2
2.5
TIME(SEC)
3
3.5
4
4.5
5
x 10
4
2
REACTIVE POWER(KVAR)
This case demonstrates the improvement of power quality of
the distribution network during grid connected operation and
compensate the harmonic injection into the distribution
network during grid connected operation. In this mode of
operation the storage battery is charge and stores the energy
for 0≤t≤0.4sec.the total load current is the sum of the
microgrid current and current supplied by the grid current.
The waveform of load current iL1 (top), current supplied by
3
0.5
4
1
0
-1
-2
-3
0
0.05
0.1
0.15
0.2
TIME(SEC)
0.25
0.3
0.35
0.4
1556
Saida and Bhavani
Grid and Islanded Operation of a Distribution Generation Inverters in a Microgrid
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.10, pp. 1553-1558
ISSN 2078-2365
http://www.ieejournal.com/
14000
4500
4000
12000
3000
REALPOWER(W)
REAL POWER(KW)
3500
10000
8000
6000
2500
2000
1500
4000
1000
2000
0
500
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0
0.1
0.4
0.15
0.2
0.25
TIME(SEC)
0.3
0.35
0.4
3500
3000
Fig.10: power supplied by MT and PV array
The remaining power is supplied by the grid is shown in the
figure.11. 7.5KW (50% of PL1) and 2KW(20% of PL2).
REAL POWER(W)
2500
2000
1500
1000
10000
500
9000
0
0.1
REAL POWER(WATTS)
8000
7000
5000
0.2
0.25
TIME(SEC)
0.3
0.35
0.4
4000
3000
2000
V. CONCLUSION
1000
0
0
0.05
0.1
0.15
0.2
0.25
TIME(SEC)
0.3
0.35
0.4
REACTIVE POWER DELIVERED BY THE GRID
1
0.8
0.6
REACTIVE POWER(KVAR)
0.15
Fig.12 Power delivered by SB and PEMFC
6000
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
0
0.05
0.1
0.15
0.2
0.25
TIME(SEC)
0.3
0.35
0.4
Fig.11 power supplied by grid
Test case 2: load sharing during Islanded operation
This test case demonstrates the islanded mode of operation
of microgrid. The circuit breaker is initially fully isolated,
when ever fault occurs on the upstream network, the circuit
breaker disconnect the microgrid from the distribution grid.
This test is employed to momentarily provide the sharing of
real and reactive power of storage battery SB and
PEMFC.during the interval of 0≤t≤0.4sec, at 0.2sec the
circuit breaker switch opened, the amount of real power
delivered by the SB and PEMFC is shown in the figure.12.at
t=0.2 sec the SB is supplies the 3.28KW and PEMFC is
supplies the 2.28KW.initially the SB is idle or charge now
in islanded mode the SB is supplies the power is shown in
the figure.12.the real power is supplied by the MT and PV
array [14]is increased
This paper presents the design of microgrid is proposed and
operation of microgrid during grid connected and islanded
mode of operation has been presented with the help of
MATLAB/SIMULINK. The proposed system integrates
with kalman filter in a control design and kalman filter is
estimate the state observer gain and extracts the harmonic
compensation in the supply voltage and load current. The
control design consists of ORC. The system is tested under
different test cases and a validate results are presented in
this paper. From these test cases the system improves the
power quality and reliability.
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Grid and Islanded Operation of a Distribution Generation Inverters in a Microgrid
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.10, pp. 1553-1558
ISSN 2078-2365
http://www.ieejournal.com/
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1558
Saida and Bhavani
Grid and Islanded Operation of a Distribution Generation Inverters in a Microgrid