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7/23/2019 Load Sharing Between Utility and Grid-connected Microgrid
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A
DISSERTATION PRESENTATION
ON
LOAD SHARING BETWEEN UTILITY AND GRID-CONNECTED MICROGRID
Guided By: Presented By:
Mr. PRAVEEN KR. AGARWAL ASHISH KR. DUBEY
Associate Professor 2009PEE109
MALVIYA NATIONAL INSTITUTE OF TECHNOLOGY, JAIPUR
DEPARTMENT OF ELECTRICAL ENGINEERING
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TOPICS TO BE COVERED
• INTRODUCTION
• PROPOSED PROBLEM AND SOLUTION
• MICROGRID TECHNOLOGIES
• POWER CONTROL METHODS FOR MICROGRID
• PROPOSED METHODOLOGY OF LOAD SHARING
• CASE STUDY AND RESULTS
• CONCLUSION
• REFRENCES
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INTRODUCTION• In distribution levels, many smaller renewable generators
(e.g. photovoltaic, fuel cells, micro hydro etc.) will beconnected to the networks. These are called distributed
generators (DGs) or distributed energy resources (DERs).
• Organized form of (DERs) and load developed the concept of
Microgrid. It has more capacity and control flexibilities tofulfil system reliability and power quality requirement.
• Here Grid-connected Mirogrid system is analyzed in different
real conditions as normal,faulty and low power generation by
microgrid subsystem and utilty.• Analysis of load flow parameter may be very helpful to
develop efficient and reliable model of grid-connected
microgrid system.
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PROPOSED PROBLEM
• A Grid-connected microgrid is an alternate for
power system reliability. But there may arise
problem like fault and lack of generation from
either side. Hence reliability of power supply
becomes major issue.
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PROPOSED SOLUTION
• To develop efficient and reliable model of grid-
connected microgrid system by calculating
load flow parameter.
• Proposed methodology analyze load
distribution in different considerd cases which
may be helpful to develop efficient and
reliable model of grid-connected microgridsystem.
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MICROGRID : A CONCEPT
Microgrid is a localized grouping of electricalgeneration,storage and loads.Some featuresare:
• Stable operation during faults and variousnetwork disturbances.
• Plug and play functionality.
•Cost-effective & environment friendly.
• Used as back-up power
• CHP(Combined heat & power)
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MICROGRID TECHNOLOGY
• Microgrids consist of several basic
technologies for operation these
include:
• distributed generation (photovoltaic,
wind, fuel cells, micro-turbines, andreciprocating internal combustion
engines with generators.).
• distributed storage(batteries, super-
capacitors, and flywheels).
•
interconnection switches, and• control Systems.
Fig. Microgrid
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CONSTRUCTION OF MICROGRID
• It consist of PV simulator, wind
simulator and battery storage
connected to the AC grid via
flexible power electronic
interface. Also there is aMicrogrid Central Controller
(MGCC) which is responsible
for the optimization of the
microgrid operation.
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Distributed generators simulator (PV Simulator)
• PV simulator is based on
the I –V curves of a PV
module.
• The PV simulator is
actually a DC voltage
source, which is
connected to the AC grid
by means of a DC-AC
inverter.
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Distributed generators simulator
(Wind energy storage system)
wind turbine can
be simulated by a
driving motorwhich is controlled
by a frequency
converter in torque
mode
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Grid-connected microgrid system
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POWER CONTROL METHODS FOR MICROGRID
1. FREQUENCY DROOP METHOD FOR
CONTROL OF LOAD SHARING :-
The conventional droop control method is
given by
w = ws – mP (1)
V = V* - nQ (2)
Where m and n are the droop
coefficients, ws is the synchronous
frequency, V is the magnitude of the
converter output voltage and w is its
frequency, while P and Q respectively
denote the active and reactive powersupplied by the converter. Thus the
frequency and the voltage are being
controlled by the active and reactive power
output of the DG sources.
2. ANGLE DROOP CONTROL :-
The average real power is denoted
by P and the reactive power by Q. These
powers, from the DG to the microgrid, can then
be calculated as
(1)
(2)
Therefore the real power can be
controlled by controlling d, while the reactive
power can be controlled by controlling voltagemagnitude.
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Power flow between Microgrid and utility
• Converter are used as power electronics interface.
• The converter is compatible in voltage and frequency with the
electric power system.
•
These power electronic interfaces provide a unique capabilityto the DG units and can enhance the operations of a
microgrid.
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• The advantage of this
structure is that power
flow can be controlled
independently in thethree phases and the
phases are magnetically
decoupled from each
other.
Contd…..
Converter structure:-
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Converter control:-
• The equivalent circuit of one
phase of the converter is
shown in Fig. In this, u×V dc1represents the converter
output voltage, where u = ± 1 .The main aim of the
converter control is to
generate u.
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PROPOSED METHODOLOGY OF LOAD
SHARING
• Methodology is proposed for load sharing
between utility and grid-connected microgrid.
• The methodology is developed for four
different cases.
• The microgrid subsystem consist of two DGs.
• Some assumption are taken for calculation
simplification.Which are following:
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Cont…
• All the connecting lines are loss less i.e. resistance of
lines are ignored.
• Voltage and angle at pcc Vp=1(pu)); δp=0 rad
• (pcc is point of common coupling between
microgrid & utility)
• In normal condition half of the total load is shared
by each side.
• Base voltage=11KV and base MVA=1MVA are
defined.
•
From utility side PTmax=1(pu), Q Tmax=1(pu)
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Cont…
Case-1:
In this case 50% load is supplied by each side.load sharing is achieved in normal condition.
Hence system operate in mode-1.
•From utility side
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Cont…
• From microgrid sideRated powers and currents are assumed in the inverse ratio of given value of
droop coefficients.Values of rated powers are so chosen that sum of rated power
of the two DGs is approaching to total active power demand by load but
somewhat lesser than total load demand.
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Cont…
• Powers shared by both DGs are
•
Angle,voltage & reactive power
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Cont…
Case-2:
•
Both DGs supply their rated load and rest ofload demand is supplied by utility.
• From Microgrid side
P=and rated values of voltage and reactive
power can be find out by using
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Cont…
• From utility side
Angle,reactive power & voltage magnitude canbe find out using
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Cont…
for this some intermediate values can be find
out using eqns. given below
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Cont…
Case-3:
DG-2 is cut off then power supplied by only DG-1,the
rated power of DG-1 is less than half of the load
demand. Hence mode-2 is invoked.
• From Microgrid side
all the value δ, V, P, Q would be rated as DG-1 is
supplying it’s rated power.
• From Utility side
• rest of load demand would be supplied
• Corresponding values of δT, Q T and VT can be
calculated using eqns. as applied in case-2.
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Cont…
Case-4:
DG-1 is cut off then power supplied by only DG-2,therated power of DG-2 is more than half of the load
demand. Hence system would operate in mode-1.
• From utility side
Half of the total power would be supplied as in
normal condition.Hence calculated values are same
as in case-1.
• From Microgrid sideDG-2 supply half of the load demand and values of δ
and V can be calculated using eqns. respectively
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Cont…
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System Data:R L= 100, LL =300mH,
base voltage=11 kV, base MVA=1,
PL =1.21 p.u. , QL =1.2838 p.u. ,
PTref =0.605, Qtref =0.6419
P1/P2= P1rated /P2rated =m2/m1=0.24/3=0.8Hence assume P1rated =0.5, P2rated =0.625
Hence assume rated current of DG1 =40A, DG2 =50A
X(transformer)=0.05(p.u.),
XL1 =0.052, XL2 =0.0415,
XLine1 =0.1021, XLine2 =0.0915,
XG =.075,
CASE STUDY AND RESULT
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Result
Case 1
Load Flow
Parameter
Utility DG-1 DG-2 Load Losses
δ 0.0432 0.026 0.0285 0 X
V 1.05 1.055 1.08 1 X
P 0.605 0.2688 0.3361 1.21 NO
Q 0.6419 0.571 0.9490 1.2838 0.8781
TABLE I
LOAD DISTRIBUTION PARAMETER UNDER CASE 1
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Result
Case 2
Load Flow
Parameter
Utility DG-1 DG-2 Load Losses
δ 0.006 0.0482 0.053 0 X
V 0.898 1.06 1.08 1 X
P 0.085 0.5 0.625 1.21 NO
Q 0.833 0.635 0.960 1.2838 1.144
TABLE II
LOAD DISTRIBUTION PARAMETER UNDER CASE 2
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Result
Case 3
Load Flow
Parameter
Utility DG-1 DG-2 Load Losses
δ 0.05 0.0482 X 0 X
V 0.915 1.06 X 1 X
P 0.71 0.5 X 1.21 NO
Q 0.854 0.635 X 1.2838 0.2052
TABLE III
LOAD DISTRIBUTION PARAMETER UNDER CASE 3
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Result
Case 4
Load Flow
Parameter
Utility DG-1 DG-2 Load Losses
δ 0.0432 X 0.051 0 X
V 1.05 X 1.085 1 X
P 0.605 X 0.605 1.21 NO
Q 0.6419 X 1.02 1.2838 0.3781
TABLE IV
LOAD DISTRIBUTION PARAMETER UNDER CASE 4
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0
0.2
0.4
0.6
0.8
1
1.2
1.4
1 2 3 4
Case 1
Case 2
Case 3
Case 4
Bus Number
Real Power
(p.u.)
Graphical representation :
Fig. Real power at four buses in considered cases
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Cont…
Fig. Voltage profile at four buses in considered cases
0
0.2
0.4
0.6
0.8
1
1.2
1 2 3 4
Case1
Case2
Case3
Case 4
Bus Number
voltage p.u.
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CONCLUSION
The conclusions are based on the work carried out and reported in the earlierchapters. The summarized conclusions of the thesis are
• In case of converter interfaced sources, power sharing can be achieved withdrooping, the output voltage angles of the converters. Angle droop controllersprovide desirable power sharing with much lower frequency deviations comparedto that of frequency droop controller.
• From study of load distribution analysis we can predict need of reserve capacityrequired in case of fault or low generation from either side.
•
Power quality of distributed generation can be improved significantly by properreference generation for the DGs. In this the compensating DG can perform loadbalancing, harmonic filtering and reactive power compensation while supplyingreal power
• The reliability in a microgrid can be improved with the application of back-to-backconverters for bidirectional power flow and voltage and frequency isolationbetween the microgrid and the utility.
•High droop gains can improve power sharing. However it can also have detrimentaleffect on system stability. A supplementary controller, which takes real power asinput, can improve the system stability significantly.
• The study of load distribution analysis may be very helpful to develop efficient andreliable model of grid-connected microgrid system.
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FUTURE SCOPE
• The angle droop control scheme can be modified to share power in a microgrid
with inertial and non inertial DG.
• Protection of back-to-back converters in case of fault in utility or microgrid faults
can be investigated.
• Improvement in supplementary droop control for enhanced system damping
under weak operating conditions. The improvement can be achieved by selectionof more appropriate input signals or controller gains.
• A modified droop control can be derived for frequency dependent loads.
• Optimal power flow control technique can be achieved
• Needed storage capacity can be determined.
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REFERENCES
• R. Majumder, A. Ghosh, G. Ledwich and F. Zare, “Power Management andPower Flow Control with Back-to-Back Converters in a Utility ConnectedMicrogrid,” IEEE transactions on power systems, vol. 25, no. 2, pp. 821-834, 2010.
• Yun Wei Li and Ching Nan Kao, “An Accurate Power Control Strategy for InverterBased Distributed Generation Units Operating In a Low Voltage Microgrid”, IEEEGeneral meeting on Energy Conversion Congress and Exposition (ECCE-2009),pp. 3363 – 3370, 2009.
• Yun Wei Li and Ching-Nan Kao, “An Accurate Power Control Strategy for Power-Electronics-Interfaced Distributed Generation Units Operating in a Low-VoltageMulti-bus Microgrid”, IEEE Transactions On Power Electronics, vol. 24, no. 12, pp.2977-2988, 2009.
• Zhe Zhang, Gengyin Li and Ming Zhou, “Application of Microgrid in DistributedGeneration Together with the Benefit Research”, IEEE Power and Energy SocietyGeneral Meeting, pp. 1 - 5, 2010.
• S. Bando, Y. Sasaki, H. Asano and S. Tagami, “Balancing control method of amicrogrid with intermittent renewable energy generators and small batterystorage”, IEEE Power and Energy Society , pp. 1 - 6 , 2008.
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Cont…
• Yanbo Che, Zhangang Yang and K.W. Eric Cheng, “Construction, Operation and
Control of a Laboratory-Scale Microgrid”, 2009 3rd International Conference on
Power Electronics Systems and Applications. International Conference
on Sustainable Power Generation and Supply, (SUPERGEN '09), pp. 1-5, 2009.
• Shervin Mizani and Amirnaser Yazdani, “Design and Operation of a Remote
Microgrid”, IEEE International Conference on Industrial Electronics (IECON-09), pp.
4299 – 4304, 2009.
• Prasenjit Basak, A. K. Saha, S. Chowdhury and S. P. Chowdhury, “Microgrid: Control
Techniques and Modeling”, Universities Power Engineering Conference (UPEC),2009, pp. 1-5, 2009.
• Wencong Su1, Zhiyong Yuan and Mo-Yuen Chow, “Microgrid Planning and
Operation: Solar Energy and Wind Energy”, IEEE Power and Energy Society General
Meeting, pp. 1-7, 2010.
• Benjamin Kroposki, Thomas Basso and Richard DeBlasio, ” Microgrid Standards and
Technologies”, IEEE Power Eng. Soc. General Meeting, pp. 1-4, 2008.
• F. Katiraei and M. R. Iravani, “Power Management Strategies for a Microgrid With
Multiple Distributed Generation Units”, IEEE Transactions On Power Systems, Vol.
21, No. 4, pp. 1821-1831, 2006.
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