- Email: [email protected]
Computer-aided design for feeders: a realistic approach
Letters
fer problems associated with windpump technology. Solutions for these problems are required if the use of windpumps is to be successfully promoted. Fundamental to any new strategy should be that it is not specific to any particular country. Once a proven design has been established the technology should be so portable, through the technology transfer process, that it can be applied to other similar countries. However, some prerequisites with regard to a minimum level of manufacturing skills may be required for any particular country to be suitable. Furthermore, minor redesign to take account of local availability of materials or re-matching of the system may be necessary as described earlier. It is important to let each organisation do what it does best. It is also vital that suitable contractual arrangements be put in place if the strategy is to achieve its objectives. Aid organisations have a vital role to play in facilitating the widespread introduction of these new second generation windpumps. They should not be deterred by mistakes of the past and neither should these mistakes be forgotten. Several crucial developments mean the time is now right for positive action. Firstly there is a new framework of international collaboration. Secondly, recent technological advances have resulted in a new and better understanding of the challenges the new technology poses, and finally the need for an effective implementation strategy, as set
out in this paper, is now gaining wider acceptance.
Computer-aided design for feeders: a realistic approach
station. A distributor on the other hand is a circuit of conductors from the distribution transformer to the service mains of the individual consumer. A ring main is adopted for feeders, forming a closed circuit having more than one feeding point. This is basically advantageous for minimising the voltage drop and losses and improving reliability and economy. It has been for quite some time the common practice to consider the voltage drop in a distributor and neglect it in the case of a feeder. Kelvin’s law was adopted for the design of feeders based on current carrying capacity and financial loss [Cotton, 1960]. This approach is not most suitable because the voltages on the secondary side are far below the required standards of electricity supply satisfying the voltage regulation. The above suggestion becomes more important when the distribution transformers are numerous and the feeders run for long distances in an open manner. In the case of distributors, the loads become heavy and the distributors are run for long distances [Manohar, 1986]. The voltages at the far end of the distributors are very low. To avoid this, ring distributors have been adopted. They form a closed circuit and have one or more feeding points. Copper conductor can be saved in this manner. One can attempt to adopt the same property for feeder circuits. Power transformers replace distribution transformers, the feeders replace the distributors and the distribution transformers replace loads when applying the same principles to feeders. On the whole the high-voltage,
C. Kumar Ratnavel Subramaniam Polytechnic, Dindigul-5, Tamil Nadu, India K. Srikrishna Department of Electrical and Electronics Engineering, Thiagarajar College of Engineering, Madurai-15, Tamil Nadu, India 1. Introduction Electrical power today plays an increasingly important role in the life of the community and development of various sectors of the economy. In every country, electrical power consumption has been continuously rising. This in turn has led to more power stations and consequent increase in power transmission lines from generating stations and the number of feeders and distributors from various sub-stations to the load centres. Transmission lines transmit power over long distances. If the transmission voltage is 275 kV and above, it is called primary transmission and if it is 66 kV to 220 kV, it is called secondary transmission [Wadhwa, 1993]. The electrical power system has two components: the feeder and the distributor. A feeder in a network is a circuit carrying power from a main sub-station to a secondary sub50
Energy for Sustainable Development
References Batchelor, S.J., and Harries, M.A.A., 1991. ‘‘An evaluation of Kijito windpumps’’, Proceedings of the European Wind Energy Conference on Wind Energy: Technology and Implementation’’, Amsterdam, pp. 861-865, Elsevier Science Publishers B.V. Bishop, N.W.M., 1991. Recorded discussion, Part II, Proceedings of the European Wind Energy Conference on Wind Energy: Technology and Implementation, Amsterdam, p. 115, Elsevier Science Publishers B.V. Burton, J.D., and Davies, D.G., 1996. ‘‘Dynamic model of a wind-driven lift pump’’, Proc. I, Mech. Eng, Vol. 210, Part A, Journal of Power and Energy, UK. Gaillard, M., and Monvois, J., 1994. ‘‘Le project Alizes en Mauritanie’’, Groupe recherché et d’Echanges Technologiques, Systèmes Solaire 100, pp. 97-107. Hacker, R.J., and Munro, D.K., 1995. ‘‘Market potential for renewable energy systems’’, Proceedings of BWEA/RAL Workshop ‘‘Technology and Implementation Issues Relating to Renewable Energy Systems in Developing Countries’’, UK, June. I.T. Power, 1994. Proceedings of International Workshop ‘‘Prospects for International Collaboration on Windpumps’’, Silsoe, UK, published by I.T. Power, UK. Padgett, B., 1995. ‘‘Commercial links with developing countries’’, Proceedings of BWEA/RAL Workshop ‘‘Technology and Implementation Issues Relating to Renewable Energy Systems in Developing Countries’’, UK, June. Polak, T.A., and Dawson, P., 1995. ‘‘Wind and water: The Poldaw windpump’’, Appropriate Technology, Vol. 2, No. 4, pp 34-35, UK, March. Smulders, P.T., 1995. ‘‘Wind water pumping -- the forgotten option’’, Proceedings of BWEA/RAL Workshop ‘‘Technology and Implementation Issues Relating to Renewable Energy Systems in Developing Countries’’, UK, June. Smulders, P.T., Burton, J.D., Pinilla, A.E., and Stacey, G., 1994. ‘‘The 3S-pump project: piston pump innovation for wind pumps’’, Paper presented at the European Wind Energy Conference (EWEC 94), Thessaloniki, Greece. UNDP, 1988. UNDP Global Wind Pump Evaluation Programme 1987-88.
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Letters
low-current feeder system resembles a low-voltage, highcurrent distribution system with these substitutions. When a feeder system in a network becomes large, the voltage drop on the feeders becomes considerable and this in turn causes low voltage on the secondary of the distribution transformers. If a small additional feeder could be installed at a reasonable cost for an open-end feeder, the whole system would become a ring feeder arrangement; the voltage drop in the feeder would become less, with the secondary of the distribution transformer voltage being maintained at rated values. A computer-aided design approach for the above system establishes the practicality of the suggestion. The investment cost on the new feeder layout is small and can be recouped in a reasonable payback period due to the minimisation of losses, and thus energy saving and cost reduction are achieved. 2. Present status Voltage drop is not a vital factor in the design of the feeder as with a distributor. In consequence a feeder is designed on the basis of current carried or on the basis of financial loss. The financial loss per annum is made up of: (a) interest on initial investment; and (b) the cost of energy wasted in the ohmic resistance of the conductor. The annual cost of energy loss must match the annual interest on investment. The above law known as Kelvin’s law was used for determining the cross-section of the conductor. The feeder cross-section as obtained in the above law is not practicable because it is too small for the safe carrying of large demand current [Cotton, 1960]. It is felt that the cross-section is small and resistance high, so the drop and loss become large. The adoption of a ring main gives low voltage drop and low losses, and offers economy. Ring mains have the following advantages [Starr, 1960] when compared with the radial main system. 1. A ring main offers a greater reliability of supply and economy of copper. 2. If a fault occurs at any feeding-point of the distributor, the continuity of the supply can be maintained. 3. The voltage at the various nodes can be maintained at stipulated values. 3. Sample system The case-study pertains to a large area called Alangulam Panchayat Union in Thirunelveli district of Tamil Nadu state in India. It has an electricity sub-station called Alangulam sub-station of capacity 6 MVA, 66/11 kV under the jurisdiction of the Tamil Nadu Electricity Board. It is connected with a 66 kV incoming feeder of the Papanasam-Rajapalayam transmission line. In the sub-station, the Alangulam feeder feeds about 96 distribution transformers of varying capacities ranging from 63 to 250 kVA, 11 kV/440V. The total installed capacity of the distribution transformers connected to the Alangulam substation feeder amounts to 9.233 MVA. The overall network is shown schematically in the single-line diagram of Figure 1. The villages benefiting from these distribution transformers are shown in Table 1. The Energy for Sustainable Development
loads in these areas are mostly domestic and commercial, some agricultural loads and a small number of industrial loads consisting of sawmills and rice-mills. No major industries exist in this area. In these systems, we are considering three tail-end transformers for our study (the dashed enclosure in Figure 1). These three transformers are situated in the village, denoted by numbers indicated in Table 2. Due to the great length of the feeder and numerous loadings en route, the tail-end suffers from poor voltage feeding. The Maranthai and Nalankattalai transformers are situated on the eastern side. With the aid of computer programming, the individual drop, total drop, the tail-end voltage and the total power loss are calculated. To aid the computer programming, a small approximation is made in the calculation of the impedance drop. Since the net value of the impedance is very nearly equal to the resistance R, the impedance drops are added arithmetically [Cotton, 1960]. To improve the tail-end voltage in the east and also to reduce the power loss, the only possible method available is to run another parallel feeder from the main sub-station to the tail-end. By this arrangement the two feeders can share the load. Since the current is shared by the two lines, the voltage drop and power loss are small and the tail will have better voltage levels. But the laying of this additional feeder involves heavy expenditure and hence it is not recommended for ring main modification. Further, the No. 3 Andipatti SS-VI, 63 kVA, 11 kV/440 V transformer is placed so that the length of the feeder is 11.99 km from Alangulam sub-station. Considering the topography, the distance between Alangulam sub-station and the Andipatti transformer is about 2.2 km as the crow flies in the southwest direction. This length is suitable for running a 11 kV feeder with 7/2.59 ACSR conductor. After connecting these two ends, a portion of the system becomes a ring main. Once again computer programming as described above is adopted for finding out the individual voltage drop, total drop and power loss for a radial feeder and a ring main feeder connected with Alangulam sub-station. Table 3 compares the voltage drop and power loss before and after the creation of the ring main. From the results, it is observed that the ring main feeder always gives better results than the radial feeder. Finally, the difference in power loss which in this case becomes a power gain in rearranging the system and the gain in revenue in terms of rupees is calculated. The total expenditure for connecting the system and the pay-back period are calculated. 4. Proposed algorithm a. Conduct a load survey of the entire Alangulam Union which is fed by the Alangulam 66 kV/11kV sub-station. Prepare a line diagram with 11 kV feeder-structures, 11 kV/440V sub-stations, the distances between poles and other electrical features. b. Data used for the calculations, like resistance and rel
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Letters
Figure 1. Existing Alangulam feeder layout. Table 1. Villages benefiting Name of the village
Symbol
No. of SS Capacity in kVA
actance of overhead conductors are noted from the standard handbook [TNEB, 1984]. Impedances of the various lines per km are found out. Assume diversity factor as 1.5. Determine current IL1, IL2, IL3, ... etc., in each section. Individual voltage drops for each section VL1, VL2, VL3 ... etc., are found out. Individual power losses for each section PL1, PL2, PL3 ... etc., are calculated. Total voltage drop and power loss are found out. After connecting it as a ring main, find out the current in each section and its direction. Determine the new voltage drops and power losses for the ring main. Compare the two values. Find out the material and construction cost for making the feeder a ring main. Determine the payback period.
Alangulam
ALM
16
2750
Aladipatti
ALP
10
952
Kuruvankottai
KK
4
363
Kurippankulam
KPK
9
978
Ayyanarkulam
AY
5
500
Sivalarkulam
SVKM
10
661
Karumpuliyuthu
KRU
5
315
Maranthai
MR
11
758
Pudur
PDR
3
300
Kallathikulam
KKLM
1
50
Nalankattalai
NLK
1
75
Puthupatti
PP
2
138
k.
Kuthapanchan
KP
8
553
5. Layout reduction
Iynthankattalai
IY
5
327
Andipatti
ADP
6
513
96
9233
Figures 1 and 2 are presented for detailed study. Figure 1 represents the existing feeder layout for the Alangulam 66 kV/11 kV sub-station. The thick line represents a 7/3.35 ACSR conductor and the thin line represents 7/2.59
Total
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Energy for Sustainable Development
c. d. e. f. g. h. i. j.
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ACSR conductors. Distances between the 11 kV/440V distribution transformers are marked in metres. The big dots in the layout represent 11 kV/440V distribution transformers in the villages connected to the Alangulam substation. For simplicity a line diagram is presented omitting the name of the sub-station, capacity in kVA of the transformer, distances between two sub-stations, etc. To illustrate further, one or two sub-stations are shown with the above relevant details. The broken-line rectangle shows the Andipatti portion under study, which is converted into a ring main circuit. Abbreviations used are as indicated in Table 1. Figure 2 shows the Alangulam sub-station and the tailend Andipatti SS-VI interconnected. Here the direction of the current and its value in amperes flowing through the various portions of the feeder are marked after connecting the abovementioned portion as a ring main. T1, T2, ..... T22 represent the tappings from the distributors. L1, L2 ..... L22 represent the lengths between two consecutive sub-stations. After the tapping T4, there is a sudden drop in current to 60.10A. The reason is that tapping T4 is a major load which consumes 147.88A. Hence, there is an enormous voltage drop and power loss as pointed out in Table 2. Further, T9 is the tapping called the point of minimum potential, where the current reversal takes place on either side.
Table 2. The sub-stations considered in the study Name of sub-station
b.
c.
Radial main Village Maranthai Tail-end voltage = Total voltage drop = Power loss = Village Nalankattalai Tail-end voltage = Total voltage drop = Power loss = Village Andipatti Tail-end voltage = Total voltage drop = Power loss = Ring main Minimum point voltage = Total voltage drop = Power loss = Comparing radial and ring main Reduction in voltage drop = Reduction in power loss = Cost of new line = Payback period =
10,243.9 V 756.0991 V 301.427 kW 10,255.03 V 744.9706 V 301.1104 kW 10,214.08 V 785.9211 V 306.7901 kW 10,700.95 V 299.0544 V 179.8378 KW 486.8667 V 126.9523 KW Rs. 245,629 2.035 months
7. Conclusion Till now, the distributor design was based on voltage drop and the feeder design on economy. The economical design provided a smaller cross-section, which caused large voltage drop and power loss with the expanding growth of the system demand. This becomes enormous in the case of a large radial feeder system. This paper has considered a very big radial feeder system in which two tail-end Energy for Sustainable Development
Capacity
Maranthai
SS-II
75 kVA
Nalankattalai
SS-I
75 kVA
Andipatti
SS-VI
63 kVA
Table 3. Comparison of radial and ring main feeders Line Length
Current (A)
Voltage drop (V)
Power loss in kW
km
Existing
Ring main
Existing
Ring main
L1
0.09
316.88
256.98
19.09
15.50
5.28
3.4
L2
0.18
315.13
255.23
37.97
30.79
10.44
6.8
L3
0.45
288.89
228.98
87.01
69.07
21.93
13.8
L4
0.90
267.89
207.98
161.37
125.50
37.72
22.8
L5
0.09
120.12
60.10
11.18
5.64
0.76
0.1
L6
0.56
107.88
47.99
62.47
27.99
6.37
1.2
L7
0.45
100.88
40.93
46.95
19.23
4.47
0.7
L8
0.15
92.13
32.10
14.29
5.05
1.24
0.1
L9
0.05
64.98
5.02
3.36
0.28
0.21
0.0
L10
1.17
51.37
8.59
62.15
9.90
3.02
0.0
L11
1.00
49.16
10.29
50.84
10.74
2.36
0.1
L12
1.62
46.54
11.42
77.97
21.80
3.43
0.2
L13
1.50
44.33
15.62
68.77
23.61
2.88
0.3
L14
0.30
42.13
17.83
30.07
5.41
0.52
0.0
L15
0.54
39.92
20.03
22.30
10.96
0.84
0.2
L16
0.72
21.45
30.50
15.97
28.93
0.32
1.0
L17
1.17
17.95
42.01
21.72
50.34
0.37
1.9
L18
0.45
14.45
45.51
6.73
20.99
0.09
0.8
L19
0.10
8.33
51.64
0.86
5.30
0.0067
0.2
L20
0.20
5.70
54.26
1.18
11.14
0.0063
0.5
L21
0.30
2.20
57.77
0.68
17.79
0.0014
0.9
L22
2.20
6. Results a.
Sub-station no.
59.97
82.71
Existing
Ring main
3.8
Existing line loss = 306.7901 kW Ring main line loss = 179.8378 kW
points remain unrealistic for ring main conversion. Another tail-end point has come in very handy for ring main conversion and the computer-aided approach has given valuable information for comparison. For a new feeder system it is better to always have a l
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Figure 2. Ring main layout with current direction shown.
ring main feeder. Even in the case of an old system a certain degree of modification for ring main conversion affords valuable gains: (1) lower voltage drop, (2) less power loss, (3) smaller pay-back period, and (4) better reliability. Thus, this paper has established that a defined computer-aided approach for a feeder system, taking into account the drop, loss and topology of the layout, provides a realistic design with very many benefits.
Cotton, H., 1960. The Transmission and Distribution of Electrical Energy, English Language Book Society, London. Manohar, V.N., 1986. ‘‘Reduction of system losses: a practical approach’’, All-India Seminar on System losses in power supply, Institution of Engineers, India, Vadodara, December. Nanda, K.S., 1995. ‘‘Energy conservation for medium industry by retrofitting HT distribution system’’, Electrical India, pp. 9-13. August. Pabla, A.S., 1992. Electric Power Distribution System, Tata McGraw-Hill, India. Starr, A.T., 1960. Generation, Transmission and Distribution of Electrical Power, Isaac Pitman & Son Pvt. Ltd., London. Tamil Nadu Electricity Board Engineers Association (TNEB), 1984. Power Engineers Handbook, Fifth edition.
Acknowledgement The authors are thankful to the authorities of the Ratnavel Subramaniam Polytechnic, Dindigul-5, and Thiagarajar College of Engineering, Madurai-15.
54
References
Energy for Sustainable Development
Wadhwa, C.L., 1993. Generation, Distribution and Utilization of Electrical Energy, Wiley Eastern Limited, New Delhi.
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fer problems associated with windpump technology. Solutions for these problems are required if the use of windpumps is to be successfully promoted. Fundamental to any new strategy should be that it is not specific to any particular country. Once a proven design has been established the technology should be so portable, through the technology transfer process, that it can be applied to other similar countries. However, some prerequisites with regard to a minimum level of manufacturing skills may be required for any particular country to be suitable. Furthermore, minor redesign to take account of local availability of materials or re-matching of the system may be necessary as described earlier. It is important to let each organisation do what it does best. It is also vital that suitable contractual arrangements be put in place if the strategy is to achieve its objectives. Aid organisations have a vital role to play in facilitating the widespread introduction of these new second generation windpumps. They should not be deterred by mistakes of the past and neither should these mistakes be forgotten. Several crucial developments mean the time is now right for positive action. Firstly there is a new framework of international collaboration. Secondly, recent technological advances have resulted in a new and better understanding of the challenges the new technology poses, and finally the need for an effective implementation strategy, as set
out in this paper, is now gaining wider acceptance.
Computer-aided design for feeders: a realistic approach
station. A distributor on the other hand is a circuit of conductors from the distribution transformer to the service mains of the individual consumer. A ring main is adopted for feeders, forming a closed circuit having more than one feeding point. This is basically advantageous for minimising the voltage drop and losses and improving reliability and economy. It has been for quite some time the common practice to consider the voltage drop in a distributor and neglect it in the case of a feeder. Kelvin’s law was adopted for the design of feeders based on current carrying capacity and financial loss [Cotton, 1960]. This approach is not most suitable because the voltages on the secondary side are far below the required standards of electricity supply satisfying the voltage regulation. The above suggestion becomes more important when the distribution transformers are numerous and the feeders run for long distances in an open manner. In the case of distributors, the loads become heavy and the distributors are run for long distances [Manohar, 1986]. The voltages at the far end of the distributors are very low. To avoid this, ring distributors have been adopted. They form a closed circuit and have one or more feeding points. Copper conductor can be saved in this manner. One can attempt to adopt the same property for feeder circuits. Power transformers replace distribution transformers, the feeders replace the distributors and the distribution transformers replace loads when applying the same principles to feeders. On the whole the high-voltage,
C. Kumar Ratnavel Subramaniam Polytechnic, Dindigul-5, Tamil Nadu, India K. Srikrishna Department of Electrical and Electronics Engineering, Thiagarajar College of Engineering, Madurai-15, Tamil Nadu, India 1. Introduction Electrical power today plays an increasingly important role in the life of the community and development of various sectors of the economy. In every country, electrical power consumption has been continuously rising. This in turn has led to more power stations and consequent increase in power transmission lines from generating stations and the number of feeders and distributors from various sub-stations to the load centres. Transmission lines transmit power over long distances. If the transmission voltage is 275 kV and above, it is called primary transmission and if it is 66 kV to 220 kV, it is called secondary transmission [Wadhwa, 1993]. The electrical power system has two components: the feeder and the distributor. A feeder in a network is a circuit carrying power from a main sub-station to a secondary sub50
Energy for Sustainable Development
References Batchelor, S.J., and Harries, M.A.A., 1991. ‘‘An evaluation of Kijito windpumps’’, Proceedings of the European Wind Energy Conference on Wind Energy: Technology and Implementation’’, Amsterdam, pp. 861-865, Elsevier Science Publishers B.V. Bishop, N.W.M., 1991. Recorded discussion, Part II, Proceedings of the European Wind Energy Conference on Wind Energy: Technology and Implementation, Amsterdam, p. 115, Elsevier Science Publishers B.V. Burton, J.D., and Davies, D.G., 1996. ‘‘Dynamic model of a wind-driven lift pump’’, Proc. I, Mech. Eng, Vol. 210, Part A, Journal of Power and Energy, UK. Gaillard, M., and Monvois, J., 1994. ‘‘Le project Alizes en Mauritanie’’, Groupe recherché et d’Echanges Technologiques, Systèmes Solaire 100, pp. 97-107. Hacker, R.J., and Munro, D.K., 1995. ‘‘Market potential for renewable energy systems’’, Proceedings of BWEA/RAL Workshop ‘‘Technology and Implementation Issues Relating to Renewable Energy Systems in Developing Countries’’, UK, June. I.T. Power, 1994. Proceedings of International Workshop ‘‘Prospects for International Collaboration on Windpumps’’, Silsoe, UK, published by I.T. Power, UK. Padgett, B., 1995. ‘‘Commercial links with developing countries’’, Proceedings of BWEA/RAL Workshop ‘‘Technology and Implementation Issues Relating to Renewable Energy Systems in Developing Countries’’, UK, June. Polak, T.A., and Dawson, P., 1995. ‘‘Wind and water: The Poldaw windpump’’, Appropriate Technology, Vol. 2, No. 4, pp 34-35, UK, March. Smulders, P.T., 1995. ‘‘Wind water pumping -- the forgotten option’’, Proceedings of BWEA/RAL Workshop ‘‘Technology and Implementation Issues Relating to Renewable Energy Systems in Developing Countries’’, UK, June. Smulders, P.T., Burton, J.D., Pinilla, A.E., and Stacey, G., 1994. ‘‘The 3S-pump project: piston pump innovation for wind pumps’’, Paper presented at the European Wind Energy Conference (EWEC 94), Thessaloniki, Greece. UNDP, 1988. UNDP Global Wind Pump Evaluation Programme 1987-88.
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low-current feeder system resembles a low-voltage, highcurrent distribution system with these substitutions. When a feeder system in a network becomes large, the voltage drop on the feeders becomes considerable and this in turn causes low voltage on the secondary of the distribution transformers. If a small additional feeder could be installed at a reasonable cost for an open-end feeder, the whole system would become a ring feeder arrangement; the voltage drop in the feeder would become less, with the secondary of the distribution transformer voltage being maintained at rated values. A computer-aided design approach for the above system establishes the practicality of the suggestion. The investment cost on the new feeder layout is small and can be recouped in a reasonable payback period due to the minimisation of losses, and thus energy saving and cost reduction are achieved. 2. Present status Voltage drop is not a vital factor in the design of the feeder as with a distributor. In consequence a feeder is designed on the basis of current carried or on the basis of financial loss. The financial loss per annum is made up of: (a) interest on initial investment; and (b) the cost of energy wasted in the ohmic resistance of the conductor. The annual cost of energy loss must match the annual interest on investment. The above law known as Kelvin’s law was used for determining the cross-section of the conductor. The feeder cross-section as obtained in the above law is not practicable because it is too small for the safe carrying of large demand current [Cotton, 1960]. It is felt that the cross-section is small and resistance high, so the drop and loss become large. The adoption of a ring main gives low voltage drop and low losses, and offers economy. Ring mains have the following advantages [Starr, 1960] when compared with the radial main system. 1. A ring main offers a greater reliability of supply and economy of copper. 2. If a fault occurs at any feeding-point of the distributor, the continuity of the supply can be maintained. 3. The voltage at the various nodes can be maintained at stipulated values. 3. Sample system The case-study pertains to a large area called Alangulam Panchayat Union in Thirunelveli district of Tamil Nadu state in India. It has an electricity sub-station called Alangulam sub-station of capacity 6 MVA, 66/11 kV under the jurisdiction of the Tamil Nadu Electricity Board. It is connected with a 66 kV incoming feeder of the Papanasam-Rajapalayam transmission line. In the sub-station, the Alangulam feeder feeds about 96 distribution transformers of varying capacities ranging from 63 to 250 kVA, 11 kV/440V. The total installed capacity of the distribution transformers connected to the Alangulam substation feeder amounts to 9.233 MVA. The overall network is shown schematically in the single-line diagram of Figure 1. The villages benefiting from these distribution transformers are shown in Table 1. The Energy for Sustainable Development
loads in these areas are mostly domestic and commercial, some agricultural loads and a small number of industrial loads consisting of sawmills and rice-mills. No major industries exist in this area. In these systems, we are considering three tail-end transformers for our study (the dashed enclosure in Figure 1). These three transformers are situated in the village, denoted by numbers indicated in Table 2. Due to the great length of the feeder and numerous loadings en route, the tail-end suffers from poor voltage feeding. The Maranthai and Nalankattalai transformers are situated on the eastern side. With the aid of computer programming, the individual drop, total drop, the tail-end voltage and the total power loss are calculated. To aid the computer programming, a small approximation is made in the calculation of the impedance drop. Since the net value of the impedance is very nearly equal to the resistance R, the impedance drops are added arithmetically [Cotton, 1960]. To improve the tail-end voltage in the east and also to reduce the power loss, the only possible method available is to run another parallel feeder from the main sub-station to the tail-end. By this arrangement the two feeders can share the load. Since the current is shared by the two lines, the voltage drop and power loss are small and the tail will have better voltage levels. But the laying of this additional feeder involves heavy expenditure and hence it is not recommended for ring main modification. Further, the No. 3 Andipatti SS-VI, 63 kVA, 11 kV/440 V transformer is placed so that the length of the feeder is 11.99 km from Alangulam sub-station. Considering the topography, the distance between Alangulam sub-station and the Andipatti transformer is about 2.2 km as the crow flies in the southwest direction. This length is suitable for running a 11 kV feeder with 7/2.59 ACSR conductor. After connecting these two ends, a portion of the system becomes a ring main. Once again computer programming as described above is adopted for finding out the individual voltage drop, total drop and power loss for a radial feeder and a ring main feeder connected with Alangulam sub-station. Table 3 compares the voltage drop and power loss before and after the creation of the ring main. From the results, it is observed that the ring main feeder always gives better results than the radial feeder. Finally, the difference in power loss which in this case becomes a power gain in rearranging the system and the gain in revenue in terms of rupees is calculated. The total expenditure for connecting the system and the pay-back period are calculated. 4. Proposed algorithm a. Conduct a load survey of the entire Alangulam Union which is fed by the Alangulam 66 kV/11kV sub-station. Prepare a line diagram with 11 kV feeder-structures, 11 kV/440V sub-stations, the distances between poles and other electrical features. b. Data used for the calculations, like resistance and rel
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Figure 1. Existing Alangulam feeder layout. Table 1. Villages benefiting Name of the village
Symbol
No. of SS Capacity in kVA
actance of overhead conductors are noted from the standard handbook [TNEB, 1984]. Impedances of the various lines per km are found out. Assume diversity factor as 1.5. Determine current IL1, IL2, IL3, ... etc., in each section. Individual voltage drops for each section VL1, VL2, VL3 ... etc., are found out. Individual power losses for each section PL1, PL2, PL3 ... etc., are calculated. Total voltage drop and power loss are found out. After connecting it as a ring main, find out the current in each section and its direction. Determine the new voltage drops and power losses for the ring main. Compare the two values. Find out the material and construction cost for making the feeder a ring main. Determine the payback period.
Alangulam
ALM
16
2750
Aladipatti
ALP
10
952
Kuruvankottai
KK
4
363
Kurippankulam
KPK
9
978
Ayyanarkulam
AY
5
500
Sivalarkulam
SVKM
10
661
Karumpuliyuthu
KRU
5
315
Maranthai
MR
11
758
Pudur
PDR
3
300
Kallathikulam
KKLM
1
50
Nalankattalai
NLK
1
75
Puthupatti
PP
2
138
k.
Kuthapanchan
KP
8
553
5. Layout reduction
Iynthankattalai
IY
5
327
Andipatti
ADP
6
513
96
9233
Figures 1 and 2 are presented for detailed study. Figure 1 represents the existing feeder layout for the Alangulam 66 kV/11 kV sub-station. The thick line represents a 7/3.35 ACSR conductor and the thin line represents 7/2.59
Total
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c. d. e. f. g. h. i. j.
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ACSR conductors. Distances between the 11 kV/440V distribution transformers are marked in metres. The big dots in the layout represent 11 kV/440V distribution transformers in the villages connected to the Alangulam substation. For simplicity a line diagram is presented omitting the name of the sub-station, capacity in kVA of the transformer, distances between two sub-stations, etc. To illustrate further, one or two sub-stations are shown with the above relevant details. The broken-line rectangle shows the Andipatti portion under study, which is converted into a ring main circuit. Abbreviations used are as indicated in Table 1. Figure 2 shows the Alangulam sub-station and the tailend Andipatti SS-VI interconnected. Here the direction of the current and its value in amperes flowing through the various portions of the feeder are marked after connecting the abovementioned portion as a ring main. T1, T2, ..... T22 represent the tappings from the distributors. L1, L2 ..... L22 represent the lengths between two consecutive sub-stations. After the tapping T4, there is a sudden drop in current to 60.10A. The reason is that tapping T4 is a major load which consumes 147.88A. Hence, there is an enormous voltage drop and power loss as pointed out in Table 2. Further, T9 is the tapping called the point of minimum potential, where the current reversal takes place on either side.
Table 2. The sub-stations considered in the study Name of sub-station
b.
c.
Radial main Village Maranthai Tail-end voltage = Total voltage drop = Power loss = Village Nalankattalai Tail-end voltage = Total voltage drop = Power loss = Village Andipatti Tail-end voltage = Total voltage drop = Power loss = Ring main Minimum point voltage = Total voltage drop = Power loss = Comparing radial and ring main Reduction in voltage drop = Reduction in power loss = Cost of new line = Payback period =
10,243.9 V 756.0991 V 301.427 kW 10,255.03 V 744.9706 V 301.1104 kW 10,214.08 V 785.9211 V 306.7901 kW 10,700.95 V 299.0544 V 179.8378 KW 486.8667 V 126.9523 KW Rs. 245,629 2.035 months
7. Conclusion Till now, the distributor design was based on voltage drop and the feeder design on economy. The economical design provided a smaller cross-section, which caused large voltage drop and power loss with the expanding growth of the system demand. This becomes enormous in the case of a large radial feeder system. This paper has considered a very big radial feeder system in which two tail-end Energy for Sustainable Development
Capacity
Maranthai
SS-II
75 kVA
Nalankattalai
SS-I
75 kVA
Andipatti
SS-VI
63 kVA
Table 3. Comparison of radial and ring main feeders Line Length
Current (A)
Voltage drop (V)
Power loss in kW
km
Existing
Ring main
Existing
Ring main
L1
0.09
316.88
256.98
19.09
15.50
5.28
3.4
L2
0.18
315.13
255.23
37.97
30.79
10.44
6.8
L3
0.45
288.89
228.98
87.01
69.07
21.93
13.8
L4
0.90
267.89
207.98
161.37
125.50
37.72
22.8
L5
0.09
120.12
60.10
11.18
5.64
0.76
0.1
L6
0.56
107.88
47.99
62.47
27.99
6.37
1.2
L7
0.45
100.88
40.93
46.95
19.23
4.47
0.7
L8
0.15
92.13
32.10
14.29
5.05
1.24
0.1
L9
0.05
64.98
5.02
3.36
0.28
0.21
0.0
L10
1.17
51.37
8.59
62.15
9.90
3.02
0.0
L11
1.00
49.16
10.29
50.84
10.74
2.36
0.1
L12
1.62
46.54
11.42
77.97
21.80
3.43
0.2
L13
1.50
44.33
15.62
68.77
23.61
2.88
0.3
L14
0.30
42.13
17.83
30.07
5.41
0.52
0.0
L15
0.54
39.92
20.03
22.30
10.96
0.84
0.2
L16
0.72
21.45
30.50
15.97
28.93
0.32
1.0
L17
1.17
17.95
42.01
21.72
50.34
0.37
1.9
L18
0.45
14.45
45.51
6.73
20.99
0.09
0.8
L19
0.10
8.33
51.64
0.86
5.30
0.0067
0.2
L20
0.20
5.70
54.26
1.18
11.14
0.0063
0.5
L21
0.30
2.20
57.77
0.68
17.79
0.0014
0.9
L22
2.20
6. Results a.
Sub-station no.
59.97
82.71
Existing
Ring main
3.8
Existing line loss = 306.7901 kW Ring main line loss = 179.8378 kW
points remain unrealistic for ring main conversion. Another tail-end point has come in very handy for ring main conversion and the computer-aided approach has given valuable information for comparison. For a new feeder system it is better to always have a l
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Figure 2. Ring main layout with current direction shown.
ring main feeder. Even in the case of an old system a certain degree of modification for ring main conversion affords valuable gains: (1) lower voltage drop, (2) less power loss, (3) smaller pay-back period, and (4) better reliability. Thus, this paper has established that a defined computer-aided approach for a feeder system, taking into account the drop, loss and topology of the layout, provides a realistic design with very many benefits.
Cotton, H., 1960. The Transmission and Distribution of Electrical Energy, English Language Book Society, London. Manohar, V.N., 1986. ‘‘Reduction of system losses: a practical approach’’, All-India Seminar on System losses in power supply, Institution of Engineers, India, Vadodara, December. Nanda, K.S., 1995. ‘‘Energy conservation for medium industry by retrofitting HT distribution system’’, Electrical India, pp. 9-13. August. Pabla, A.S., 1992. Electric Power Distribution System, Tata McGraw-Hill, India. Starr, A.T., 1960. Generation, Transmission and Distribution of Electrical Power, Isaac Pitman & Son Pvt. Ltd., London. Tamil Nadu Electricity Board Engineers Association (TNEB), 1984. Power Engineers Handbook, Fifth edition.
Acknowledgement The authors are thankful to the authorities of the Ratnavel Subramaniam Polytechnic, Dindigul-5, and Thiagarajar College of Engineering, Madurai-15.
54
References
Energy for Sustainable Development
Wadhwa, C.L., 1993. Generation, Distribution and Utilization of Electrical Energy, Wiley Eastern Limited, New Delhi.
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