gbghnhn
Post on 04-Jun-2018
215 Views
Preview:
TRANSCRIPT
-
8/13/2019 gbghnhn
1/6
IEEE TRANSACTIONS ON SMART GRID, VOL. 4, NO. 1, MARCH 2013 353
Advanced Power Distribution System Configurationfor Smart Grid
Jae-Chul Kim, Member, IEEE, Sung-Min Cho, Member, IEEE, and Hee-Sang Shin
AbstractPower distribution systems should meet demandssuch as high reliability, efficiency, and penetration of renewableenergy generators (REGs) in a smart grid. In general, power dis-
tribution systems are radial in nature. One-way power flow is theadvantage of a radial system. However, the introduction of REGs
causes bidirectional power flow. Furthermore, there are limitsto improvements in reliability and efficiency in a radial system.Therefore, the upgrading of primary feeders from a radial to aloop configuration has been considered in the Korea Smart Distri-
bution Project. An advanced power distribution system (APDS),
in which primary feeders operate in a loop configuration, has beenexplored in this paper. First, the design scheme of a conventionalpower distribution system configuration that adopts distribution
automation is introduced. Subsequently, an upgrading scheme ofloop configuration using normally opened tie switches and a tie
switch selection algorithm for loss minimization are described.
Finally, the advantages of the upgraded configuration are reportedthrough case studies. It is observed that the APDS configurationcan integrate more REGs from the viewpoint of voltage regulation.An advanced distribution system allowing greater use of REGswill be a major contribution to smart grid implementation.
Index TermsPower distribution system configuration, renew-able energy generation, smart distribution, voltage regulation.
I. INTRODUCTION
C ONCERNS about global climate change have increasedthe penetration of REGs, which are connected to powerdistribution systems. The Korean government announced a na-
tional objective to increase the share of REGs, which was 2.24%
in 2006, to 11% by 2030. In general, power distribution sys-
tems are radial in nature. In addition, given their relatively late
development, REGs were not considered in the design phase
of current power distribution systems. For these reasons, con-
necting distributed generation units (i.e., REGs) to the distri-
bution system may cause various interconnection problems in-
cluding harmonic concerns, system overvoltage, fault coordina-
tion, increased fault currents, and islanding concerns. To miti-
gate these problems, the IEEE Std. 1547 for distributed gener-ator (DG) interconnection was published in 2003 [1]. In Korea,
Manuscript received November 26, 2012; accepted December 09, 2012. Dateof publication February 06, 2013; date of current version February 27, 2013.This work was supported by the Power Generation & Electricity Delivery ofthe Korea Institute of Energy Technology Evaluation and Planning (KETEP)grant funded by the Korea government Ministry of Knowledge Economy (No.2009T100200067). Paper no. TSG-00821-2012.
S.-M. Cho (corresponding author) is with the Department of Electrical Engi-neering, Soongsil University, Seoul 156-743, Korea (e-mail: dannyone@ssu.ac.kr).
J.-C. Kim and H.-S. Shin are with the Department of Electrical Engi-neering, Soongsil University, Seoul 156-743, Korea (e-mail: jckim@ssu.ac.kr;shs8828@ssu.ac.kr).
Digital Object Identifier 10.1109/TSG.2012.2233771
TABLE ICAPACITYLIMITS OFDG [2]
KEPCO (Korea Electric Power Cooperation) also published a
DG interconnection manual in 2005. In this manual, the capacity
of a DG is limited by the voltage level and line configuration.
Table I shows capacity limits of a DG [2].
The main goal of these DG installment limitations is to keep
the distribution system voltage within a permissible range. In
Korean power distribution systems, the distribution voltage is
controlled by an on-load tap changer (OLTC) through the line
drop compensation (LDC) method. In some cases, the high pen-
etration of REGs may cause voltage regulation failures [3].
A fault occurrence in a feeder or lateral that is connectedto an REG leads to its interruption. For stable reclosing oper-
ation and maintenance crew safety, the REGs must detect is-
landing operationand be disconnected within 0.5 s according to
the KEPCO DG interconnecting manual. Furthermore, REGs
should wait 5 min to re-connect after the distribution system
becomes stable [1], [2]. Therefore, the reliability of the power
distribution system is important for REGs as well as for cus-
tomers.
For several decades, the configuration of power distribu-
tion systems has been typically designed in a radial form for
easy control [4], [5]. However, some power utilities such as
Taipower, Florida Power Company, Hong Kong Electric Com-
pany, and Singapore Power have adopted normally closed loop
configurations to serve their customers with high reliability
[6][9]. Loop power distribution systems have the advantages
of reliability and voltage regulation. To accommodate these
strengths, an APDS including loop configuration is being
developed in the Korea Smart Distribution System Project.
In Korea, KEPCO has already adopted a distribution automa-
tion system (DAS) with many normally opened tie switches to
improve reliability [10]. Therefore, without feeders or lateral
extensions, the power distribution system can simply be up-
graded to an APDS through tie switch closure. However, to op-
erate in loop configuration, the protection system should be up-
graded as well. All reclosers and circuit breakers (CBs) in the
1949-3053/$31.00 2013 IEEE
-
8/13/2019 gbghnhn
2/6
354 IEEE TRANSACTIONS ON SMART GRID, VOL. 4, NO. 1, MARCH 2013
Fig. 1. Conventional power distribution configuration in KEPCO.
loop path should detect bi-directional fault currents. Thus, ap-
propriate tie switches are selected to upgrade the radial configu-
ration to a loop configuration with the minimum upgrading cost.
The purposeof this paper is to introduce the advantages of the
APDS, which is being studied in the Korea Smart Distribution
Project, and to propose an algorithm for appropriate tie switch
selection. In section two, the conventional power distribution
system in Korea is described. The basic scheme of an APDS and
the algorithm for tie switch selection are introduced in section
three. In section four, the advantages of the APDS are examined
through a case study.
II. CONVENTIONALPOWERDISTRIBUTIONCONFIGURATION
A. Design Scheme Which Considers Reliability
The DAS is very useful for reliability enhancement. To min-
imize interruption time, KEPCO has adopted the DAS for de-termination of fault location, fault isolation, and service restora-
tion. A conventional power distribution configuration adopting
the DAS is shown in Fig. 1.
In the figure, CB, RA, and GA are substation circuit breakers,
automatic reclosers, and remote-controlled switches, respec-
tively. Distribution feeders are normally divided into three
sections. Each section has one or more normally opened tie
switches for service restoration. For example, if a fault occurs
in section two as illustrated in Fig. 2, automatic equipment de-
vices which experience a fault current generate a fault indicator
(FI) signal within 30 s. At the same time, RA1 is opened to clear
the fault. Subsequently, by considering the FI message, thefault location is determined. Next, GA3 and GA4 are opened to
isolate the fault. Finally, RA1 and GA10 or GA6 are closed for
service restoration. As these processes are completed within 5
min, the DAS is very useful in improving reliability. However,
because the REG interconnected in section three experiences
momentary interruptions, it should be disconnected within 0.5
s by an anti-islanding detection function. Furthermore, the
REGs should wait 5 min to re-connect after service restoration
is complete [2], [10] .
In the conventional power distribution system, a service
restoration scheme using normally opened tie switches is useful
for minimizing interruption time. However, REG operation is
sensitive to momentary interruptions due to anti-islanding de-
tection. Therefore, the service restoration process in an APDS
Fig. 2. Diagram of a conventional LDC voltage regulation method.
Fig. 3. Diagram of a conventional LDC voltage regulation method.
should reduce momentary interruptions to assure continuous
operation of REGs.
B. Voltage Regulation Scheme
Power utilities are required to keep customers voltage pro-
files on feeders close to therated value under all load conditions.
In urban areas, the power transformer located in the substation
with an under-load tap changer (ULTC) is the main voltage reg-
ulation equipment. Pole-mounted voltage regulators (PVRs) are
additionally installed in rural feeders. A ULTC controlled by the
LDC method is used to keep the voltage constant at a fictitious
regulation point (FRP) regardless of the magnitude or power
factor of the load. Sending currents and voltages , as
shown in Fig. 3, are used to calculate the FRP voltage according
to (1). The power distribution system operator should set up pa-
rameters such as dead band, time delay, reference voltage, resis-tance , and reactance for automatic voltage regulation
[3][5].
(1)
where
Fictitious regulation point (FRP) voltage.
Sending voltage.
Sending current.
Resistance of a feeder from ULTC to FRP.
Reactance of a feeder from ULTC to FRP.
-
8/13/2019 gbghnhn
3/6
KIMet al.: ADVANCED POWER DISTRIBUTION SYSTEM CONFIGURATION FOR SMART GRID 355
In Korean power distribution systems, a power transformer
bank has 6 to 8 feeders. The voltage of the feeders depends on
the ULTC at the power transformer. If the load imbalance be-
tween feeders exceeds an acceptable range, voltage regulation
using only ULTC may fail. Furthermore, if many REGs are con-
nected to the feeders, LDC detects sending currents that are less
than the actual values. The output of REGs depends on weather
conditions such as wind, solar radiation, and temperature, and is
therefore uncontrollable. For this reason, it becomes more diffi-
cult for the distribution system operator to predict the load bal-
ance in advance as the penetration of REGs increases. There-
fore, high penetration of REGs may cause voltage regulation
failures [3].
III. ADVANCEDPOWERDISTRIBUTIONCONFIGURATION
A. Basic Scheme
The configuration of a conventional power distribution
system is radial because of its simplicity. However, as men-tioned above, the upgrading of primary feeders is needed
because there are numerous problems for a radial structure to
accommodate many REGs. In Korea, primary feeders have
at least three normally opened tie switches. By closing the
opened tie switches, the radial distribution configuration can
be upgraded to a loop structure without installing additional
electric power lines. Fig. 3 shows the upgrading scheme of an
ADPS, which includes a loop feeder structure as an example.
If tie switch GA4 is closed, feeder1 and feeder2 form a loop
structure. In this case, other tie switches should be opened to
avoid a mesh structure. Although additional electric power
lines do not need to be installed, the protection devices shouldbe upgraded to operate the power distribution system in a
loop structure. Therefore, the optimum tie switches should be
selected to maximize the profits of ADPS upgrading for the
loop structure.
B. Loop Configuration Selection Algorithm for Loss
Minimization
Load imbalance between feeders increases loss in the power
distribution system. In a radial structure, network reconfigura-
tion is used for load imbalance alleviation, loss minimization
and others [11]. However, in a loop structure, a loop path con-
necting a heavily loaded feeder and a lightly loaded feeder canalleviate the load imbalance to minimize loss.
The voltage drop in a heavily loaded feeder is larger than that
in lightly loaded feeders. Therefore, by considering the voltage
across the opened tie switch, we can infer which side of the
tie switch is the heavily loaded feeder. If the voltage across an
opened tie switch is high, loop operation using the tie switch
is more effective for loss minimization. Therefore, we present
a loop path selection algorithm using the voltage across open
tie switches. The loop path selection algorithm considers a 24 h
load profile because the output is for representing stable rather
than temporary states. A flowchart of the loop path selection
algorithm for loss minimization is shown in Fig. 4.
The loop path selection algorithm is as follows.
Step 1) Generate tie switch cases for loop configuration.
Fig. 4. Flowchart of loop path selection algorithm for loss minimization.
Step 2) Carry out powerflow analysis for 24 h.
Step 3) Calculate accumulated switch voltage (ASV) for all
tie switches according to (2).
(2)
where
Tie switch number.
Hours.
Open voltage of switch n at hour h.
Step 4) If all ASV calculations for tie switches are com-
pleted, calculate the total ASV according to (3) for
each case.
(3)
Step 5) Select the case with the maximum total ASV for loss
minimization.
IV. CASESTUDY
In this section, we compared the conventional radial structure
and the APDS loop structure to explore the advantages of the
APDS from the perspective of loss reduction and voltage profile.
The conventional Korean power distribution system adopting
the DAS, which is shown in Fig. 5, is used as the test distribu-
tion system model [12]. In the test model, there are four feeders
and six normally opened tie switches. There are three available
cases for upgrading the primary feeder from a radial to a loop
configuration. All test cases are summarized in Table II. Case 1
-
8/13/2019 gbghnhn
4/6
356 IEEE TRANSACTIONS ON SMART GRID, VOL. 4, NO. 1, MARCH 2013
Fig. 5. Conventional Korea power distribution system adopting DAS.
TABLE IITEST CASES FORCASESTUDY
Fig. 6. Load profile of each feeder in the test distribution system model.
involves a radial structure and cases 2, 3, and 4 involve a loop
structure.
Each feeder supports electric power of various loads for res-
idential, commercial, and industrial customers. Most load pro-
files depend on the type of customer. Fig. 6 shows feeder load
profiles in the test model. Feeder 3 shows a relatively light load
throughout the day. Alternatively, feeders 1 and 2 show a heavy
load. The load profiles are derived from actual load patterns in
Korea.
Fig. 7. Losses during 24 h in the test distribution system model.
A. Loss Reduction Study
Powerflow analysis was performed for loss analysis using the
cases summarized in Table II. Fig. 7 shows the losses accumu-
lated during 24 h for each case. Upgrading to case 3, in which
the loss is reduced from 6.74 to 6.42 MWh, is the best solution
for loss minimization. However, to compare loss reduction be-
tween cases, powerflow analysis should be conducted for eachcase. Yet if there are more tie switches, the processing time will
increase. Therefore, to compare loss reduction more efficiently,
we applied the proposed loop path selection algorithm to test the
power distribution system model. In the radial case, the ASVs
for each tie switch calculated by (2) are shown in Fig. 8.
The total ASVs for each case are summarized in Table III.
The total ASV is highest in case 3. Therefore, this case is the
best loop-upgrading solution for loss minimization. This shows
that we can select a loop-upgrading path without conducting
powerflow analysis for each case.
B. Voltage Control Study
Voltage regulation in the test power distribution system
model mainly depends on ULTC control by the LDC method.
-
8/13/2019 gbghnhn
5/6
KIMet al.: ADVANCED POWER DISTRIBUTION SYSTEM CONFIGURATION FOR SMART GRID 357
Fig. 8. ASV for each tie switch.
TABLE IIICALCULATEDTOTAL ASV
Fig. 9. Maximum and minimum voltage profiles for case 1 and case 3.
Maximum and minimum voltage profiles for cases 1 and 3 arecompared in Fig. 9. The difference between the maximum and
minimum voltages for case 3 is narrower than that for case 1
(radial). Therefore,voltage regulation is easier in case 3.
The voltage regulation of power distribution systems in
which many REGs are interconnected may fail. Therefore, we
compared the robustness of voltage regulation between cases
1 and 3. The REGs were interconnected at line sections F25,
F36, and F414, respectively. Subsequently, we changed the
generation capacity from 1 to 10 MW. The maximum and
minimum voltages in the test power distribution system are
shown in Fig. 10. In case 1, undervoltage occurred because
the LDC method failed as a result of the 24 MW generated bythe REGs. In contrast, in case 3, the power distribution system
could accommodate the 30 MW generated by the REGs within
a permissible voltage range.
V. CONCLUSION
In this paper, we analyzed the advantages of an APDS loop
structure from the perspective of loss reduction and voltage reg-
ulation. In addition, we presented a loop path selection algo-
rithm for loss minimization. The results of case studies using
the test power distribution system are summarized as follows:
1) Appropriate upgrading of primary feeders from a radial to
a loop configuration reduces loss in the power distribution
system
Fig. 10. Maximum and minimum voltages with REGs of case 1 and case 3.
2) The robustness of voltage regulation in the loop structure
allows the APDS to accommodate more REGs than the
radial structure.
3) The proposed loop path selection algorithm gives the best
solution for loss minimization using only a power flow
analysis of the radial system.
Coordination of protection using communication technology
may permit the upgrading of primary feeders from radial to loop
configurations. We expect the APDS presented here to support
a more suitable environment for high penetration of REGs in
smart grids.
APPENDIX
The line impedance and load profiles of the test power dis-
tribution system used in the case study are listed in Tables IV
and V, respectively. All per units are based on 22.9 kV and 100
MVA. All power factors are 0.9.
TABLE IVLINE IMPEDANCE
-
8/13/2019 gbghnhn
6/6
358 IEEE TRANSACTIONS ON SMART GRID, VOL. 4, NO. 1, MARCH 2013
TABLE VAVERAGE LOAD AT END OFLINE SECTION
REFERENCES
[1] IEEE Standard for Interconnecting Distributed Resources With Elec-tric Power Systems, IEEE Std. 1547-2003.
[2] Korea Electric Power Co.,A Guide to the Distributed Generation In-terconnection to Distribution System 2005.
[3] J.-H. Choi and J.-C. Kim, Advanced voltage regulation method ofpower distribution systems interconnected with dispersed storage andgeneration systems (Revised), IEEE Trans. Power Del., vol. 16, no.2, pp. 329334, Apr. 2001.
[4] T. A. Short, Electric Power DistributionHandbook. Boca Raton, FL,USA: CRC, 2004.
[5] T. Gonen, Electric Power Distribution System. New York, NY, USA:McGraw-Hill, 1986.
[6] B. Pagel, Energizing international drive,Transm. Distrib. World, pp.
1834, Apr. 2000.
[7] T. C. Yu, Prin ciples and Design of Low Voltage S ystems. Singapore:Byte Power Publ., 1996, pp. 1314.
[8] W. T. Huang, T. H. Chen, G. C. Pu, Y. F. Hsu, and T. Y. Guo, As-sessment of upgrading existing primary feeders from radial to nor-mally closed loop arrangement, inProc. 2002 IEEE Power Eng. Soc.Transm. Distrib. Conf., pp. 21232128.
[9] T. H. Chen, W. T. Huang, J. C. Gu, G. C. Pu, Y. F. Hsu, and T. Y. Guo,Feasibility study of upgrading primary feeders from radial and open-loop to normally closed-loop arrangement, IEEE Trans. Power Syst.,vol. 19, no. 3, pp. 13081316, 2004.
[10] N.-G. James, Robert Wilson, Control and Automation of ElectricalPower Distribution Systems. Boca Raton, FL, USA: CRC, 2006.
[11] S. M. Cho, H. S. Shin, J. H. Park, and J. C. Kim, Distribution systemreconfiguration consideringcustomer and DG reliability cost,J. Elect.
Eng. Technol., vol. 7, no. 4, Jul. 2012.[12] H. T. Lee, A study on the reliability analysis of loop power distribu-
tion systems with microgrid structure, Ph.D. dissertation, Dept Electr.Eng., Soongsil Univ., Seoul, Korea, 2009.
Jae-Chul Kim (M84) received the B.S. degree from Soongsil University,Korea, in 1979, and M.S. and Ph.D. degrees from Seoul National University,Korea, in 1983 and 1987, respectively.
He hasbeena professor of ElectricalEngineering at Soongsil University since1988. His research interests include power system reliability, smart distributionsystems, and smart grids.
Sung-Min Cho(S08M13) received the B.S., M.S. and Ph.D. degrees in elec-trical engineering from Soongsil University, Korea, in 2003, 2008 and 2012, re-spectively.
Currently, he is a Postdoctoral Researcher with Soongsil University. His re-searchinterests include powersystem reliability, smart distribution systems, anddistributed generation interconnection.
Hee-Sang Shin received the B.S. and M.S. degrees in electrical engineeringfrom Soongsil University, Korea, in 2007 and 2009, respectively. Currently, heis working toward the Ph.D. degree at Soongsil University Graduate School.
His research interests include smart distribution systems, and electric railways.
top related