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International Journal of Ambient Energy Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/taen20
A combined practical approach for distribution system loss reduction a
a
b
Duong Quoc Hung , N. Mithulananthan & R.C. Bansal a
School of Information Technology and Electrical Engineering, The University of Queensland, Brisbane Qld 4072, Australia b
Department of Electrical, Electronic and Computer Engineering, University of Pretoria, South Africa Accepted author version posted online: 09 Aug 2013.Published online: 24 Sep 2013.
To cite this article: Duong Quoc Hung, N. Mithulananthan & R.C. Bansal , International Journal of Ambient Energy (2013): A combined practical approach for distribution system loss reduction, International Journal of Ambient Energy, DOI: 10.1080/01430750.2013.829784 To link to this article: http://dx.doi.org/10.1080/01430750.2013.829784
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International Journal of Ambient Energy, 2013 http://dx.doi.org/10.1080/01430750.2013.829784
A combined practical approach for distribution system loss reduction Duong Quoc Hunga , N. Mithulananthana and R.C. Bansalb∗ a School
of Information Technology and Electrical Engineering, The University of Queensland, Brisbane Qld 4072, Australia; b Department of Electrical, Electronic and Computer Engineering, University of Pretoria, South Africa (Received 23 November 2012; accepted 25 July 2013 )
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This paper investigates the loss reduction potential of primary distribution systems with distributed generation (DG) units, capacitors and network reconfiguration. A methodology is proposed to combine different types of DG units and capacitors for minimising losses. A combination of reconfiguration, DG and capacitor placement to obtain high loss reduction is also presented. The results obtained on a 33-bus test distribution system with constant load condition and a 687-bus real distribution system with varying load conditions show significant loss reduction using the proposed approach. The combination of type 1 DG (i.e. DG capable of delivering real power only) and capacitor placement or reconfiguration with type 1 DG and capacitor placement can be viable for minimising losses. Keywords: capacitor; distributed generation; distribution system, loss reduction; reconfiguration
1. Introduction Reconfiguration and capacitors have been traditionally utilised for loss reduction, voltage enhancement and system capacity release in distribution systems. In recent years, the high penetration of DG units in distribution systems as a result of fuel cost price hike, liberalisation of electricity markets and environmental concerns have created significant changes in the existing distribution system structures. Consequently, planning and operation strategies of distribution systems need to be modified to accommodate DG units. It is widely acknowledged that strategically placed and operated DG units can yield several benefits such as loss reduction, voltage and reliability enhancement, and network upgrade deferral. On the other hand, improper placement and operations of DG units in some circumstances may reduce benefits and even jeopardise the existing systems. Moreover, DG placement alone without considering reconfiguration and capacitors is unlikely to obtain an optimal solution. Loss minimisation in distribution systems has been focused on optimising reconfiguration and capacitor placement over the past decades. Several optimisation techniques for network reconfiguration such as heuristic algorithm (Ababei and Kavasseri 2011), genetic algorithm (GA) (De Macedo Braz and De Souza 2011) and particle swarm optimisation (PSO) (Abdelaziz et al. 2009) have been presented. Like reconfiguration, a wide range of optimisation techniques have been employed for capacitor placement such as heuristic algorithm (Hamouda and Zehar 2011), GA (Taher, Hasani, and Karimian 2011) and PSO (Taher, Karimian, ∗ Corresponding
author. Email:
[email protected]
© 2013 Taylor & Francis
and Hasani 2011). Similarly, a number of studies on DG placement have been reported using techniques such as analytical approaches (Acharya, Mahat, and Mithulananthan 2006; Hung, Mithulananthan, and Bansal 2010; Hung and Mithulananthan 2013; Hung and Mithulananthan 2012), heuristic algorithm (Abu-Mouti and El-Hawary 2011a), PSO (AlRashidi and AlHajri 2011) and artificial bee colony (ABC) (Abu-Mouti and El-Hawary 2011b). The loss reduction benefits from reconfiguration with capacitors have been widely accepted by the research community. Many optimisation techniques for this work such as simulated annealing (Su and Lee 2001), GA (Guimaraes, Castro, and Romero 2010) and ant colony search (ACS) (Chang 2008) have been presented. Recently, reconfiguration with DG units has been investigated using PSO (Olamaei, Niknam, and Gharehpetian 2008) and ACS (Yuan-Kang et al. 2010). The combination of DG and capacitor placement has been introduced using ABC (Abu-Mouti and El-Hawary 2011b). The contribution of this paper is to propose a methodology to combine different types of DG units and capacitors for minimising losses using analytical expressions (Hung, Mithulananthan, and Bansal 2010). A combination of reconfiguration, DG and capacitor placement to obtain high loss reduction is also presented in this paper. The rest of the paper is structured as follows: Section 2 introduces a methodology for DG and capacitor placement using analytical expressions. Reconfiguration with DG and capacitor placement is also explained in this section. Section 3 portrays the 33-bus test system and the 687-bus
2
D.Q. Hung et al.
real system along with numerical results and interesting discussions. The comparative study between the proposed approach and another found in the literature is provided in Section 4. Finally, the key contributions and conclusions of the work are summarised in Section 5. 2. Proposed methodology 2.1. Power loss The total real power loss in a distribution system with N buses as a function of active and reactive power injection in all buses can be calculated as (Elgerd 1971). Ploss =
N N
[αij (Pi Pj + Qi Qj ) + βij (Qi Pj − Pi Qj )],
i=1 j=1
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(a) Type 1 DG unit (pfDG = 1): is capable of injecting active power only. (b) Type 2 DG unit (pfDG = 0): is capable of injecting reactive power only. However, this type of DG may not be economically viable in terms of reactive power injection. Hence, capacitors should be used rather than the type 2 DG unit. (c) Type 3 DG unit (0 < pfDG < 1 and sign = +1, i.e. lagging power factor): is capable of injecting both real and reactive power. (d) Type 4 DG unit (0 < pfDG < 1 and sign = −1, i.e. leading power factor): capable of injecting real power and absorbing reactive power.
(1) where αij = (rij /Vi Vj ) cos(δi − δj ); βij = (rij /Vi Vj ) sin (δi − δj ); Vi ∠δi is the complex voltage at the ith bus; rij + jxij = Zij is ijth element of impedance matrix [Zbus ]; rij and xij are, respectively, the branch resistance and reactance between buses i and j; Pi and Pj are active power injections at the ith and jth buses, respectively; Qi and Qj are reactive power injections at the ith and jth buses, respectively.
2.2.2. Sizing capacitor at various locations When pfDG is set to be zero, the optimal size of capacitor for each bus for minimising the system power loss can be obtained from Equation (2) as follows:
QCi = QDi −
N 1 (αij Qj + βij Pj ). αii j=1
(3)
j=i
2.2. Analytical expressions 2.2.1. Sizing DG unit at various locations The optimal size of each DG unit for each bus i for minimising the power loss can be expressed as follows (Hung, Mithulananthan, and Bansal 2010): ⎧ αii (PDi + ai QDi ) − Xi − ai Yi ⎨P , DGi = a2i αii + αii (2) ⎩ QDGi = ai PDGi , where
ai = ± tan φi = ± tan(cos−1 (pfDGi )),
(αij Pj − βij Qj ), Yi =
N
Xi =
N j=1 j=i
(αij Qj + βij Pj ); pfDGi is the oper-
j=1 j =i
ating power factor of the DG unit at bus i; ai is positive for DG injecting reactive power and ai is negative for DG consuming reactive power; PDGi and QDGi are the real and reactive power injection from the DG unit at bus i respectively; PDi and QDi are the real and reactive power of load at bus i, respectively. The power factor of the DG unit depends on the operating conditions and adopted DG types. The optimal size of each DG type at each bus i for the system loss to be minimum can be determined from Equation (2) by setting the value of pfDG in the following different manners (Hung, Mithulananthan, and Bansal 2010). DG technologies such as biomass-based synchronous machines can fall under the following types depending on their control strategy and real and reactive power capability.
2.3.
Computational procedures
2.3.1. DG and capacitor placement The computational procedure to place DG and/or capacitor units is explained below. This procedure has been reported for DG placement in Hung and Mithulananthan (2013). When the value of pfDG is set to be zero, the computational procedure for capacitor placement is considered.
Step 1 Run load flow for the base case and find losses using Equation (1). Step 2 Enter the power factor of the DG unit. Step 3 Find the optimal location of the DG unit using the following steps. (a) Calculate the optimal size of the DG unit at each bus using Equation (2). (b) Place the DG unit with the optimal size as mentioned earlier, at each bus one at a time. Calculate the loss for each case using Equation (1). (c) Locate the optimal bus at which the loss is the lowest. Step 4 Find the optimal size of DG unit and calculate losses using the following steps. (a) Place a DG unit at the optimal bus obtained in Step 3(c), change this DG size in a small step, and calculate the loss for each case using Equation (1) by running load flow. (b) Select and store the optimal size of the DG unit that gives the lowest loss.
International Journal of Ambient Energy
3
Step 5 Check constraints: Stop if either the following occurs: (a) the voltage violation takes place at any bus; (b) the total size of DG units is larger than the total system demand plus loss; (c) the maximum number of DG units is unavailable; (d) the new iteration loss is greater than the previous iteration loss.
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The previous iteration loss is retained; otherwise, repeat Steps 2–5. For comparison, the following options are considered in the above computational procedure. The numerical results of these options are provided in Section 3. • Option 1: Only capacitor addition (C); • Option 2: Only DG addition (DG); • Option 3: Capacitor placement is carried out before DG addition (C + DG); • Option 4: DG placement is carried out before capacitor addition (DG + C). 2.3.2. Reconfiguration with DG and capacitor placement This section considers reconfiguration with DG and capacitor placement. The Power System Simulator and Advanced Distribution Engineering Productivity Tool (PSS/ADEPT) is adopted for reconfiguration. The combination of DG and capacitor placement is explained in Section 2.3.1. From the planning perspective, reconfiguration should be carried out before DG and/or capacitor addition. For comparison, four options are considered as follows: • Option 1: Only reconfiguration (R); • Option 2: Reconfiguration is carried out before capacitor addition (R + C); • Option 3: Reconfiguration is carried out before DG addition (R + DG); • Option 4: Reconfiguration is carried out before DG addition and then capacitors are added after placed DG units (R + DG + C). 3. Numerical results 3.1. 33-Bus test system with constant load condition The proposed methodology has been applied to a 12.66 kV test distribution system with constant load condition. This system has 33 buses, 5 tie lines and a total load of 3.7 MW and 2.3 MVAr, as depicted in Figure 1. The complete data of this system are given in Venkatesh, Ranjan, and Gooi (2004). The proposed methodology has been developed and simulated in MATLAB environment. The lower and upper voltage thresholds are 0.95 and 1.05 p.u., respectively. Dispatchable biomass DG units are
Figure 1.
The 33-bus test distribution system.
considered in this study. It is assumed that a maximum of three DG units and three capacitors can be placed in the system. However, the proposed methodology can consider the larger number of DG units and capacitors. The provision of reactive power from DG units is certainly more costly than capacitors. Hence, in order to maximise real power injection from DG units and minimise the DG investment cost as expected by DG owners, the combination of type 1 DG and capacitor placement is recommended in this simulation. In addition, the results of type 3 DG units combined with capacitors and type 4 DG units with capacitors are also provided for comparison. 3.1.1. DG and capacitor placement Table 1 presents the simulation results of type 1 DG and capacitor (C) placement for the 33-bus system. Based on the proposed methodology, the results of 12 cases considered are as follows: 1 C, 2 Cs, 3 Cs, 1 DG, 2 DGs, 3 DGs, 1 C + 1 DG, 2 Cs + 2 DGs, 3 Cs + 3 DGs, 1 DG + 1 C, 2 DGs + 2 Cs and 3 DGs + 3 Cs. The results include the optimal size and location of DG units and capacitors with the corresponding loss reduction (Ploss ) for each case. It can be seen from Table 1 that the highest loss reduction is 90.28% when three capacitors and three DG units are installed. In contrast, the lowest loss reduction is 28.26% with one-capacitor case.
4 Table 1. Options Base 1. C
D.Q. Hung et al. Type 1 DG and capacitor placement for the 33-bus system.
Base 1C 2 Cs 3 Cs
2. DG
1 DG 2 DGs 3 DGs
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3. C + DG
1 C + 1DG
2 Cs + 2DGs
3 Cs + 3DGs
4. DG + C
Schedule of installed DG units and capacitors
Cases
1 DG + 1C
2 DGs + 2Cs
3 DGs + 3Cs
Bus Size Bus Size Bus Size Bus Size Bus Size Bus Size Bus Size Bus Size Bus Size Bus Size Bus Size Bus Size Bus Size Bus Size Bus Size Bus Size Bus Size Bus Size
30 1250 30 1100 30 1000 6 2601 6 1800 6 900 30 1250 6 2601 30 1000 6 1800 30 1000 6 900 6 2601 30 1250 6 1800 30 1000 6 900 30 1000
12 440 12 500 14 720 12 900
DG (kW)
C (kVAr)
Ploss (kW)
Ploss (%)
–
–
211.20
0.00
1250
151.52
28.26
1540
142.03
32.75
2000
138.61
34.37
2601
111.10
47.39
2520
91.63
56.61
2520
81.05
61.62
58.55
72.28
32.76
84.49
20.52
90.28
1250
58.55
72.28
1500
32.76
84.49
2000
20.53
90.28
24 500
31 720
1250 2601 12 500 14 720 12 500 12 900
1500 2520 24 500 31 630
2000 2430 2601
14 720 12 500 12 900 12 500
It can be observed from Table 1 that when type 1 DG units are considered, both options (Option 3: DG units added after capacitors and Option 4: DG units added before capacitors) can produce almost the same result of optimal locations, sizes and loss reductions. However, when type 3 DG units with 0.85 lagging power factor are considered, the optimal location and size of DG units between both Options 3 and 4 are considerably different, as shown in Table 2. The optimal size and location of capacitors between both options are also significantly different as illustrated in the above tables. Consequently, a difference in optimal loss reduction results between both options can be observed. When type 4 DG units with 0.85 leading power factor are analysed, remarkable differences in the results of optimal locations, sizes and loss reductions between both options can be found in Table 2. It can also be seen from the results that the combination of type 1 DG units and capacitors can be a feasible solution for loss reduction, while satisfying
2520 31 720 24 500
2520
the primary purpose of maximising real power supply from DG units as compared to other cases. The next section provides simulation results on reconfiguration with type 1 DG and capacitor placement. In this option, reconfiguration is carried out before DG and capacitor placement as proposed in the methodology. 3.1.2. Reconfiguration with DG and capacitor placement Table 3 shows the simulation results of reconfiguration (R) with type 1 DG and capacitor (C) placement for 33-bus system. Based on the proposed methodology, the results of 10 cases considered are as follows: R, R + 1 C, R + 2 Cs, R + 3 Cs, R + 1 DG, R + 2 DGs, R + 3 DGs, R + 1 DG + 1 C, R + 2 DGs + 2 Cs and R + 3 DGs + 3 Cs. The results include tie-lines (lines switched out) and the optimal sizes and locations of DG units and capacitors with the corresponding loss reduction for each case. Before
International Journal of Ambient Energy Table 2.
Types 3 and 4 DG and capacitor placement for the 33-bus system.
Options 3. C + DG
Cases
DG Power factor (type)
3 Cs + 3 DGs
0.85 lag (type 3)
0.85 lead (type 4)
4. DG + C
3 DGs + 3 Cs
0.85 lag (type 3)
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0.85 lead (type 4)
Table 3.
5
Schedule of installed DG units and capacitors Bus Size Bus Size Bus Size Bus Size Bus Size Bus Size Bus Size Bus Size
30 1000 6 1059 30 1000 6 1176 6 710 31 300 10 450 26 800
12 500 14 847 12 500 12 941 30 710 24 300 24 450 30 800
DG (kVA) 24 500 31 635 24 500 30 706 13 710 25 300 26 450 10 540
C (kVAr)
Ploss (kW)
Ploss (%)
24.31
88.49
24.43
88.43
900
20.28
90.40
2140
62.92
70.21
DG (kW)
C (kVAr)
Ploss (kW)
Ploss (%)
– –
– –
211.20 121.83
0.00 42.31
1125
92.77
56.08
1870
84.65
59.92
2050
83.45
60.49
1425
82.76
60.81
2850
51.64
75.55
2925
46.81
77.84
1125
55.53
73.71
1815
17.31
91.80
2000
11.71
94.46
2000 2541 2000 2823 2130
1350
Reconfiguration with type 1 DG and capacitor placement for the 33-bus system.
Options Base 1. R 2. R + C
3. R + DG
4. R + DG + C
Cases Base R R+1C
33-34-35-36-37 7-9-14-28-31 7-9-14-28-31
R + 2 Cs
7-9-14-28-31
R + 3 Cs
7-9-14-28-31
R + 1 DG
7-9-14-28-31
R + 2 DGs
7-9-14-28-31
R + 3 DGs
7-9-14-28-31
R + 1 DG + 1 C
7-9-14-28-31
R + 2 DGs + 2 Cs
R + 3 DGs + 3 Cs
Schedule of installed DG units and capacitors
Tie-lines
7-9-14-28-31
7-9-14-28-31
Bus Size Bus Size Bus Size Bus Size Bus Size Bus Size Bus Size Bus Size Bus Size Bus Size Bus Size Bus Size
reconfiguration is carried out, the system has five tie-lines: 33, 34, 35, 36 and 37. After the system is reconfigured using the reconfiguration tool (Power System Simulator and Advanced Distribution Engineering Productivity Tool (PSS/ADEPT)), five new tie lines found are 7, 9, 14, 28 and 31. It can be observed from Table 3 that reconfiguration with three DG units and three capacitors can yield
30 1125 30 1100 30 1000 29 1425 29 1425 29 1300 29 1425 30 1125 29 1425 30 1100 29 1300 30 1000
21 770 21 750 8 1425 8 1300
6 300
32 325
1425 8 1425 8 715 8 1300 21 700
2850 32 325 6 300
2925
the highest loss reduction of 94.46%, while reconfiguration only can produce the lowest loss reduction of 42.31%. 3.1.3. Results of bus voltages Table 4 shows the minimum and maximum bus voltages in all cases for the 33-bus system. Even up to three capacitors
6 Table 4.
D.Q. Hung et al. Voltages (p.u.) in all cases with type 1 DG units for the 33-bus system.
Options Base R C DG DG + C R+C
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R + DG R + DG + C
Cases
Min. voltage @ bus
Max. voltage @ bus
Base R 1C 2 Cs 3 Cs 1 DG 2 DGs 3 DGs 1 DG + 1 C 2 DGs + 2 Cs 3 DGs + 3 Cs R+1C R + 2 Cs R + 3 Cs R + 1 DG R + 2 DGs R + 3 DGs R + 1 DG + 1 C R + 2 DGs + 2 Cs R + 3 DGs + 3 Cs
0.9037 @ 18 0.9434 @ 32 0.9164 @ 18 0.9298 @ 18 0.9319 @ 18 0.9425 @ 18 0.9539 @ 33 0.9690 @ 18 0.9545 @ 18 0.9820 @ 33 0.9866 @ 25 0.9438 @ 32 0.9543 @ 32 0.9540 @ 32 0.9443 @ 32 0.9650 @ 32 0.9768 @ 32 0.9447 @ 32 0.9840 @ 32 0.9889 @ 32
1.0000 @ 1 1.0000 @ 1 1.0000 @ 1 1.0000 @ 1 1.0000 @ 1 1.0000 @ 1 1.0000 @ 1 1.0000 @ 1 1.0000 @ 1 1.0030 @ 14 1.0078 @ 12 1.0000 @ 1 1.0000 @ 1 1.0000 @ 1 1.0000 @ 1 1.0000 @ 1 1.0000 @ 1 1.0009 @ 29 1.0042 @ 8 1.0034 @ 21
are inserted in both systems, the bus voltages are still under the lower limit of 0.95 p.u. Either reconfiguration or DG placement can produce better voltage enhancement than capacitor placement. Moreover, reconfiguration with capacitors or reconfiguration with DG units or the combination of DG units and capacitors are employed, the bus voltages enhance significantly within acceptable limits. More importantly, the voltage profiles become perfect when the combination of 3-DG and 3-capacitor placement or reconfiguration with 3-DG and 3-capacitor allocation is considered. It is interesting to note that the voltage profiles improve substantially when the number of DG units and/or capacitors installed in the systems is increased. Overall, it can be found from the simulation results in both test systems that the combination of type 1 DG units and capacitors or reconfiguration with type 1 DG units and capacitors could be viable for minimising losses and enhancing voltage profiles. The proposed method has been applied to a 687-bus real distribution system with varying load conditions as a practical case study, as described in the next section. 3.2. 687-Bus real system with varying load conditions The proposed methodology has been applied to a 687-bus Vietnamese utility distribution system with five tie-lines (Hung 2008). The system is supplied from a 115/23 kV 40 MVA substation with a total load demand of 15.46 MW and 7.31 MVAr. In this system, three varying load levels in a year are considered as follows: 0.5 p.u. (light load level) for a period of 1000 h, 0.75 p.u. (medium load level) for a period of 6760 h and 1.0 p.u. (peak load level) for a period of 1000 h. Three type 1 DG units with a unity power factor
and three capacitors are placed at each load level. Here, the biomass-based DG unit is considered as a dispatchable source. In this context, its output can be adjusted. The fixed and switched capacitor banks are considered. Fixed capacitors operate on the feeder all the time, while switched capacitors are turned ‘on’ or ‘off’, depending on the load level (Bala, Kuntz Jr., and Pebles 1997). The following parameters are employed: the discrete standard-sizes of DG units and capacitor banks are multiples of 0.5 MW and 0.3 MVAr, respectively; the DG output can be dispatched in a small step of 0.5 MW; the lower and upper voltage thresholds are set to be 0.95 and 1.05 p.u. In this case, the computational procedure described in Section 2.3 has been performed for each load level and the method of energy loss calculation in Hung and Mithulananthan (2012) has been adopted. Here, the energy loss for each load level can be calculated as the power loss times the duration (period) at that load level. Subsequently, the total annual energy loss can be calculated as the sum of all energy losses at all load levels. After reconfiguration with DG and capacitor placement is carried out, the best solution can be obtained where the total annual energy loss of the system is the lowest without violations of any constraints as given in Section 2.3. Table 5 shows the initial system performance for each load level. The power losses are relatively high at 493.62 kW for the peak load level and 269.03 kW for the medium load level, along with low voltages at some buses under 0.95 p.u. The power loss at the light load level is 116.07 kW. The total annual energy loss of the system is relatively high at 2.43 GWh. The proposed methodology has been applied for each load level. Before reconfiguration is carried out, the system has five tie-lines (lines switched out): 687, 688, 689, 690 and 691. After the system
International Journal of Ambient Energy Table 5.
System before and after reconfiguration with three DG units and three capacitors for the 687-bus system. Real loss (kW)
Load level
Base case system 1 493.62 0.75 269.03 0.5 116.07 Total annual loss System with R + 3 DGs + 3 Cs 1 119.97 0.75 67.92 0.5 30.33 Total annual loss
Min. voltage in p.u. @ bus
Max. voltage in p.u. @ bus
Annual loss (MWh)
0.9019 @ 548 0.9282 @ 552 0.9533 @ 548
1.0000 @ 1 1.0000 @ 1 1.0000 @ 1
493.62 1818.64 116.07 2428.33
0.9580 @ 176 0.9688 @ 175 0.9794 @ 174
1.0012 @ 618 1.0003 @ 618 1.0006 @ 210
119.97 459.14 30.33 609.44
Table 6. Optimal locations and sizes of DG units and capacitors for the 687-bus system. Optimal bus
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7
1
0.75
0.5
DG output dispatch schedule (MW) 618 3 2 1.5 210 1.5 1 1 374 1.5 1 1 Capacitor dispatch schedule (MVAr) 618 1.5 1.2 0.6 210 0.9 0.6 0.3 21 0.6 0.3 0.3
Optimal size (MW) 3 1.5 1.5 1.5 0.9 0.6
is reconfigured, five new tie lines considering all load levels found are 38, 154, 335, 359 and 600. Table 5 also presents the results of the combined case (R + 3DGs + 3 Cs). After the system is reconfigured, three DG units and three capacitors are optimally added in the system, significant loss reductions found are 119.97 kW for the peak load level, 67.92 kW for the medium load level, and 30.33 kW for the light load level. The bus voltages for each load level improve significantly within the acceptable limits. The overall annual energy loss of the system is considerably reduced to 0.61 GWh that corresponds to a reduction level of 74.90%. Table 6 shows the optimal location and size of DG units and the corresponding dispatch schedule of the DG output for each load level. As DG units are dispatchable sources, their outputs vary according to the corresponding load levels so that the system loss for each load level as well as the overall annual system energy loss considering three load levels remain at the lowest level. In this case, the optimal locations for three different load levels remain identical. The optimal sizes of three DG units are 3 MW at bus 618, 1.5 MW at bus 374 and 1.5 MW at bus 210 at the peak load level. It can also be observed from the results that significant decrease in DG outputs are found at the light and medium load levels compared to the peak load level. For example, the DG outputs at bus 618 are 3, 2 and 1.5 MW for the peak, medium and light load levels, respectively.
Table 6 also presents the optimal location and size of capacitors and the corresponding dispatch schedule for each load level. At bus 618, two capacitor banks (0.3 MVAr each, fixed type) at all load levels, two banks (0.3 MVAr each, switched type) at the medium and peak loads, and one bank (0.3 MVAr, switched type) at the peak load can be installed. At bus 210, one bank (0.3 MVAr, fixed type) at all load levels, one bank (0.3 MVAr, switched type) at the medium and peak loads, and one bank (0.3 MVAr, switched type) at the peak load can be inserted. Similarly, at bus 21, one bank (0.3 MVAr, fixed type) at all load levels and one bank (0.3 MVAr, switched type) at the peak load can be added. It can be observed from the results that significant decrease in the amount of capacitor capacity is found at the medium and light load levels compared to the peak load level so that the system losses for these load levels remain at minimum level. Figure 2 shows the annual energy loss reduction trend obtained by the proposed methodology for the 687-bus real system considering various cases (3 Cs, R, R + 3 Cs, 3 DGs, R + 3 DGs, 3 DGs + 3 Cs and R + 3 DGs + 3 Cs). This loss reduction trend for this system is similar to that found in the 33-bus test system. Reconfiguration can yield a loss reduction that is higher than the three capacitor case, but lower than the three DG unit case. Significant loss reduction is found in all combined cases including DG units that are superior to the other cases in the absence of DG units. The highest energy loss is 2.09 GWh when three capacitors are installed in the system. In contrast, the lowest energy loss is 0.61 GWh that corresponds to a loss reduction of 74.98%, when the combined case of R + 3 DGs + 3 Cs is considered. The second lowest energy loss is 0.73 GWh with a corresponding loss reduction of 69.96%, when the combination of three DG units and three capacitors is considered.
4. Comparative study Table 7 presents the results of the combination of the type 3 DG unit and capacitor placement on the 69-bus distribution systems with constant load condition (Baran and Wu 1989). The DG power factor is pre-specified at 0.85 lagging. Based
8
D.Q. Hung et al.
Annual loss (GWh)
3 2.09
2 1.24
1.12
1.06
1
0.76
0.73
R+3DG
3DG+3C
0.61
0
3C
Figure 2.
R
R+3C
R+3DG+3C
Comparison of energy loss results of different cases for the 687-bus system.
Table 7. Results of type 3 DG and capacitor placement by different approaches for the 69-bus system. Approach
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3DG
DG power DG C Ploss factor Bus (MVA) (MVAr) (%)
ABC (Abu-Mouti and El-Hawary 2011b)
Lag. 0.85
61
2.2
Proposed
Lag. 0.85
18 61 17
2.2
91.75 0.3 0.3
91.76
on the proposed method, the system that is placed with one 2.2 MVA DG unit at bus 61 and one 0.3 MVAr capacitor bank at bus 17 can produce a loss reduction of 91.76%. The loss reduction result obtained by ABC (Abu-Mouti and El-Hawary 2011b) is 91.75%, when the system is inserted with one 2.2 MVA DG unit at bus 61 and one 0.3 MVAr capacitor bank at bus 18. In this case, the optimal location of capacitor bank obtained by the proposed approach (bus 17) is different from the result achieved by ABC (bus 18).
5. Conclusions This paper has proposed a methodology to combine different types of DG units and capacitors for minimising losses. A combined methodology of reconfiguration, DG and capacitor placement has also been presented. The results show that the combination of type 1 DG and capacitor placement or reconfiguration with type 1 DG and capacitor placement could be feasible for loss reduction. When type 1 DG placement is carried out before or after capacitor addition, both options can produce almost the same result of optimal locations, sizes and loss reduction. However, when types 3 and 4 DG units are considered, significant differences in the results have been observed. It is worth mentioning that reconfiguration can reduce power losses significantly. Capacitors only contribute a small proportion to loss reduction, but have a significant impact on voltage enhancement when they are combined with DG units or reconfiguration or both. DG units can
make a considerable contribution to loss reduction and voltage profile enhancement as the introduction of DG units into the systems can reduce the active and reactive power flows from the source to the location of DG units. All combined cases that include DG units are superior to other cases excluding DG units from a loss-reduction perspective. It is interesting to note that reconfiguration with three DG units and three capacitors can produce a power loss reduction of 94.46% for the 33-bus test system with constant load condition and an annual energy loss reduction of 74.90% for the 687-bus real system with varying load conditions. Significant voltage profile enhancement has been observed in all these cases.
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