load sharing between utility and grid-connected microgrid

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



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



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.



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.



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.



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)



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



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.



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.



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



Grid-connected microgrid system



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.



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.



• 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:-



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.



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:



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)



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



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.



Cont…

• Powers shared by both DGs are

•

Angle,voltage & reactive power



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



Cont…

• From utility side

Angle,reactive power & voltage magnitude canbe find out using



Cont…

for this some intermediate values can be find

out using eqns. given below



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.



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



Cont…



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



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



Result

Case 2

Load Flow

Parameter


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




Result

Case 3

Load Flow

Parameter


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




Result

Case 4

Load Flow

Parameter


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




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



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.



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.



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.



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.



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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load sharing between utility and grid-connected microgrid

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