Arab J Sci Eng (2014) 39:3023–3031DOI 10.1007/s1336901309225
RESEARCH ARTICLE  ELECTRICAL ENGINEERING
Minimization of Power Losses in Distribution System viaSequential Placement of Distributed Generationand Charging Station
J. J. Jamian
·
M. W. Mustafa
·
H. Mokhlis
·
M. A. Baharudin
Received: 13 September 2012 / Accepted: 15 December 2012 / Published online: 29 January 2014© King Fahd University of Petroleum and Minerals 2014
Abstract
Energy saving in a distribution system throughminimization of power losses is a crucial task for costeffective operation. One of the most effective approaches in minimizing losses is through the application of distributed generation (DG). To achieve minimum losses, the correct sizeof DG could be determined using various optimization techniques. Different from other approaches, this paper presentsan alternative method to minimize losses through optimalsize of DG and optimal location of charging station (CS),which overall reduces the power losses of the network. Thispaperalsoinvestigatestheimpactofconstantvoltage(PV)orconstant power (PQ) mode of DG to the overall power losseswith the aim of choosing the best mode. The suitable DGoperation modes and proper sequential placement betweenDG and CS does not only reduce the power losses, it hasalso opened new possibilities to add new additional loads inthe system. The algorithm’s performance shows promisingresults in minimizing power losses when tested using 33 busradial distribution networks.
Keywords
Charging station
·
Distribution generation
·
Loss reduction
·
Optimization
·
Optimal new load placement
J. J. Jamian (
B
)
·
M. W. Mustafa
·
M. A. BaharudinFaculty of Electrical Engineering, Universiti Teknologi Malaysia,Johor Bahru, Malaysiaemail: jasrul@fke.utm.myM. W. Mustafaemail: wazir@fke.utm.myM. A. Baharudinemail: mariff@fke.utm.myH. MokhlisFacultyofEngineering,UniversityofMalaya,KualaLumpur, Malaysiaemail: hazli@um.edu.my
1 Introduction
Thehighpenetrationofdistributedgeneration(DG)indistribution network has provided many opportunities to improvethe efﬁciency of distribution network. With proper coordination of DG the distribution network will have minimumpower losses and at the same time, open new opportunitiesto add new loads near to the consumer side. Many attemptsof using optimization methods to allocate the DG into thedistribution network have been proposed either for a single DG unit [1–3] or using multiple DG units [4–6]. The
analysis in [7] is one of the recent studies on optimal placementofmultipleDGunits.Theauthorusedamultiobjectiveoptimization analysis on the system by considering various types of load models that exist in the network suchas industrial load, residential load, mixed load and others.Besides that, the authors have also analyzed the impact of
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3024 Arab J Sci Eng (2014) 39:3023–3031
DG on the most sensitive bus in the network. In addition,[8] also used the voltage sensitive analysis of the line forthe determination of the suitable location for multiple DGunits as voltage supporters. The authors used the DG as atool to improve the voltage proﬁle. This concept has alsobeen implemented in [9] where GA was applied in determining the suitable DG location for improving the voltagestability.TheobjectiveofDGsizingisnotonlylimitedtothereductionofpowerlossesandloweringthecostofgeneration[10], but the sizing of DG can also be used to minimize thetotal harmonic distortion (THD) in the network [11], controlling the stability margin of the network [12] and many
more.Apart from improving the voltage proﬁle, the optimallocation of DG units can also be used to minimize the lossesin the network [13,14]. However, these losses can be further
lowered by ﬁnding the optimal size of DG. Thus, most of theresearchers tend to make adjustments using the modern optimizationmethodtoﬁndtheidealsizeofDGaftertheoptimalplacement has been achieved. The study in this manuscript,ontheotherhandproposesanovelapproachtoensurethatthesystem operates optimally with minimum power losses afterthe optimal location for DG has been obtained. By maintaining the initial size of the DG (to maximize their output), theproper placement of charging station (CS) modeled as a constant current load (CCL) in the system can help to improvethe efﬁciency of the distribution network. Thus, this strategy (allocate DG and load) is very useful for power utilities in identifying suitable location for DG and CS that canminimize the power losses.The CCL load type is chosen in this study, since it wellrepresenting the operation characteristic of CS for the electrical vehicle. In other words, the CS will be located in thesystem after the installation of DG for two main purposes: tohelp the system to operate with minimum power losses andto support the growth of electric vehicle (EV) in the future.Thus, the main contribution of this research is to determinethe optimal locations of DG unit as well as CS in the system, so that the power losses’ impacts on the distributionnetwork can be minimized. After the allocation of the DGunit and its power output, the proposed algorithm for optimal CS location is used to utilize the existing DG output andmake the distribution system to be more efﬁcient. Furthermore,attheendofthisresearch,thecomparisonbetweenCSallocations based on the proposed method is compared withrandom location to prove the effectiveness of the proposedmethod.This paper is organized as follows: The problem description is presented in Sect. 2 along with description of somemain components of the power systems used in this study.The mathematical model and proposed algorithm for optimal sequential placement between DG and CS in the distribution network is then deﬁned in Sect. 3. Section 4 shows
the simulation results in terms of voltage proﬁle at each busin the network, power losses reduction due to DG allocationand also the impact of allocation of CS in power systems.Last but not least, Sect. 5 presents the conclusions of thestudy.
2 Characteristic of DG and CS Units
Distributed generation can operate as PQ or PV bus depending on the ability of the DG in producing a constant voltage. In [15], the authors have described in detail this issueand have categorized the type of DG based on their ability.In this study, by assuming that the power system plannerhas a choice to choose either PV or PQ mode, the analysis on the impact of these two types of mode to the distribution network is studied in terms of voltage proﬁle andpower losses. For the DG allocation process, only a single unit of DG is assumed to be placed at a bus, since itis not practical to allocate multiple units of DG at the samebus.As mentioned in the Sect. 1, the CCL load has been chosen in this analysis to represent the CS for electrical vehicle.This is due to the charging process, which draws a constantcurrent until it reaches 80 % of the state of charging (SoC).Presently, there are three types of CS that are being usedin some countries, which are the CS levels I, II and III. Allthese CSs are categorized based on the voltage level andthe voltage source types (either AC or DC) for the chargingprocess. The CS levels I and II use AC/DC approaches tocharge the EV with the voltage level around 100 and 230 V,respectively. Although the CS for levels I and II are usingthe standard voltage value to charge the EV, it requires avery long time to complete the process (few hours). On theother hand, the CS level III (or also known as fast charging method) uses a different approach to charge the EV,which is using the DC/DC converter. The CS level III isthe faster charging process compared to the other two CSs.However, it causes some drawbacks to the distribution network, such as larger reactive power consumption and voltage drop. In summary, all these charging methods have theirown advantages and disadvantages, either in terms of thecharging performance or impact to the distribution system[16–18].
In this study, the charging AC level II method is usedin the analysis. Suppose the maximum power for a singlecharging slot is 7.7 kW. With the assumption that each CSconsist of 10 units per charging point and always drawing maximum power, each unit of CS consumes 77 kWpower from the distribution network. Thus, the rated powerdemand (
P
o
−
CCL
)
for the CS in this study is equal to 77 kW,with the constant current value of 320 A at voltage level of 230 V.
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3 Allocation of DG and CS Units in DistributionNetwork
The process to determine the DG output in this study beginsby ﬁnding the optimal location for DG and followed by theallocationofCSstooptimizetheoperationsystem(minimumpower loss).3.1 Mathematical Modeling for DG AllocationFrom the initial study [19], the optimal location for DG canbe obtained by ﬁnding the maximum value of loss savingwhere:
S
i
=−
n
i
=
1
(
2
D
i
I
2
ai
I
DG
i
+
D
i
I
2DG
i
)
R
i
(1)where
i
line number in system,
D
i
1, if DG located at thebranch, else 0,
I
ai
line real active current,
I
DG
i
DG
i
th busreal active current,
R
i
line resistance,
S
i
i
th bus loss savingvalue.The formula for loss saving is derived from the differencebetween the initial power losses and the new power lossesafter the DG is connected to the network. The DG current(
I
DG
)
value is calculated based on (2) for each bus except
the slack bus. By ﬁnding the
I
DG
for each bus, the savinglosses (
S
i
)
caused by the possible DG connection can beobtained.
I
DG
=−
ni
=
1
D
i
I
ai
R
i
ni
=
1
R
i
(2)The bus which has maximum loss saving value is chosen asthe best location to allocate the DG. The size of DG can beobtained by multiplying the I
DG
with the bus voltage valuefor that particular bus. By allocating the selected DG intothe network and assuming it as a base system, the load ﬂowanalysiswillbeexecutedagaintoﬁndthenextlocationofDGuntil the stopping criteria is attained. However, the authorsin [19] only considered the PQ bus types (negative load) of
DG.Inthisstudy,thecomparisonbetweenDGoperatinginPVmode and DG operating in PQ mode are discussed and thebest results between them are used in this analysis.3.2 Mathematical Modeling for CS AllocationThe presence of CS unit in the network will also affect theamount of current ﬂowing in the branch. Since the CS isalways in constant current mode, the new active current thataffects the amount of power losses in the branches is:
I
new
ai
=
I
ai
+
I
CS
(3)
StartSolve Load Flow For Existing NetworkDetermine Optimal Placement And Capacity For DG Unit Using (1)(2) Substitute / Installed The DG Unit Based At The Optimal Location Next DG Capacity = Negative Value?Determine Next DG LocationDetermine Optimal Location For CS Unit In The NetworkNumber Of CS Units > 2 At Selected Location?Select 2
nd
lower Min P
incr
(5) in the systemNumber Of CS = Max? Print Number Of DGs With Their Capacities And CS LocationsEndSolve Load Flow Analysis For The Current Network
NoNoYESYESYESNo
Fig. 1
Process to coordinate DG and CS in the distribution system
The new power loss that occurs at the branch where CS isconnected can be derived as the following equation:
P
lossesnew
=
D
i

I
new
ai

2
R
i
=
D
i
(
I
ai
+
I
CS
)
2
R
i
=
D
i
(
I
2
ai
+
2
I
CS
I
ai
+
I
2CS
)
R
i
(4)where
I
CS
real active current consumed by CS.The objective functions to determine the best location forCSisbasedontheminimumincrementvalueofpowerlossesin the network after the CS is connected to the line/branch,where:
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3026 Arab J Sci Eng (2014) 39:3023–3031
Fig. 2
The 33 bus radialdistribution network
23456789101112131415161718192021222627282930313233232425
S
u
bstation132/12.66kV
Fig. 3
Voltage proﬁle fordistribution network
0.940.950.960.970.980.991.001.01123456789101112131415161718192021222324252627282930313233
V o l t a g e i n p . u
Bus Number
DG as PV busDG as PQ bus
Min
P
incr
=
P
lossesnew
−
P
lossesinitial
=
D
in
i
=
1
(
I
2
ai
+
2
I
CS
I
ai
+
I
2CS
)
R
i
−
I
2
ai
R
i
=
D
in
i
=
1
I
2CS
R
i
+
2
I
CS
I
ai
R
i
(5)and subject to:
V
min
(
0
.
95p
.
u
)
≤
V
i
≤
V
max
(
1
.
05p
.
u
)
(6)0
≤
Cap
(
DG
j
)
≤
Cap
max
DG
j
(7)Thus, from formulae (1) to (7), the DG and CS units are
located in the distribution network based on the proposedalgorithm to improve the power losses value in the system.Figure 1 shows the analysis and the process for determiningthe best location for CS to obtain the optimized distributionsystemoperationaftertheoptimalDGallocationprocesshasbeen completed. From the ﬁgure, the maximum number of DG units allowed in the system is limited by the negativevalue of DG capacity. It means that, at the negative valuecondition, the DG is not operating as a generator, but operating as a motor in the system. Once this value is reached, theprocess is stopped.After the number of DG and its value is obtained, the CSunits will play a role in optimizing the existing DG outputin the system. By using a formula that has been discussedfor optimal CS allocation (5), one by one CS is placed at the
appropriatelocationsuntilreachingthemaximumnumberof CS (in this case study, maximum CS
=
6 units). Each busis assumed to have only two CS units (maximum per bus).WhenthebushasalreadycontainedtwounitsofCS,thenextminimumpowerincrement[Min
P
inc
—formulain(5)]valueis chosen as the optimal location.
4 Results and Discussion
The proposed algorithm is tested using 33 bus radial distribution system to determine the optimal location for DGand also CS as shown in Fig. 2. The system line data and
load data can be obtained from [20]. For this system, the
power base value is 10 MVA and the voltage base value is12.66 kV.
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