THE THREE VOLTAGE LEVEL DISTRIBUTION USING THE 1 V LOW VOLTAGE SYSTEM Juha Lohjala. Suur-Savon Sähkö Ltd. Finland. juha.lohjala@sssoy.fi Tero Kaipia, Jukka Lassila, Jarmo Partanen. Lappeenranta University of technology. Finland tero.kaipia@lut.fi, jukka.lassila@lut.fi, jarmo.partanen@lut.fi INTRODUCTION The need to improve the quality and economy of the electricity distribution process has increased year by year. An interesting new innovation in the distribution network development has been the use of 1 V () low voltage lines together with 2 kv and.4 kv systems. The 1 V low voltage distribution system gives an opportunity to answer to the demand of ever-growing need to improve the economy and quality of electricity distribution business. It is based on old principles that today are possible to be taken into use both economically and technically. The EU-legislation enables the use of 1 V low voltage level as a third distribution voltage level between the current medium voltage network and the low voltage network. Adding the third voltage level shortens the length of the medium voltage network and diminishes the number of short branches and affects the interruption costs of the entire distribution network. In this paper the basic structure of new 2/1/.4 kv system is illustrated. Paper presents the basics of operating and protecting the 1 V low voltage network in boundaries given by the European standardising. Techno-economic analyses of the usability of the three voltage level distribution system are introduced as well as some case examples of the three voltage level distribution system already used in Suur-Savon Sähkö distribution company located in the Eastern Finland. The three voltage level distribution system has been in use at lake district for several years. The experiences are very promising. TECHNICAL DESCRIPTION OF THE 1 V DISTRIBUTION SYSTEM The boundaries of low voltage are defined in the first article of the EU low voltage directive LVD 73/23/EEC. For alternating voltage the range of voltage level is from 5 to 1 V and for direct voltage from 75 to 15 V. According to the directive, instruments that are classified for these voltage ranges are low voltage instruments [1]. The 1 V low voltage is used between the 2 kv medium voltage network and.4 kv low voltage network as shown in Fig. 1. 2 kv a) b) Fig. 1 Example network topologies for feeding customers with a) traditional and b) three voltage level distribution systems. 2 kv As presented in Fig. 1, using the 1 V voltage level makes it possible to reduce the length and number of branches in medium voltage network. This, especially in the overhead line network, diminishes the possibility of blackouts for the entire medium voltage line and so affects to interruption costs and the quality of distribution. Based to theoretical examinations and measurement results from the experimental installations of Suur-Savon Sähkö Ltd (later SSS Ltd) the 1 V part of the distribution network is operated as non-grounded. Usually the low voltage network is operated as grounded. The safety regulations define that the voltage between the ground level and the zero wire of the system cannot exceed 75 V in any part of the low voltage network during any possible fault situation [2]. In common Finnish grounding circumstances this rule is almost impossible to fulfil with the 1 V system if neutral is grounded. The.4 kv low voltage network starting from 1/.4 kv distribution transformers is operated as grounded [3]. When the 1 V network is operated as non-grounded, protection is executed with relays and circuit breakers. Then the only limit for the length of the 1 V line comes from voltage drop of the used cables. The overcurrent and short circuit protection is carried out with current breakers very similar to ones normally used in today s low voltage networks. For earth fault protection in the 1 V network, the direction of the fault current does not have to be known. The earth fault can then be detected by measuring the potential between the star point and ground of the system. In SSS Ltd. potential between the star point of the 2/ transformers and ground is measured. The measurement technique of the neutral potential is simple. Only one voltage transformer between star point and ground potential is needed. The principle of earth fault protection is presented in Fig. 2.
L1 L2 L3 Fig. 2 U1 M K 1 K 2 Earth fault protection arrangements of the 1 V distribution system. M = voltage transformer, K 1 = time lock, K 2 = trip relay. The protection system is more expensive compared to normal fuse protection. However, the advantage of the system is that it does not restrict the length of the 1 V line like fuse protection. In practise the used protection system is integrated in one package and can be installed for example to a pole like pole-fuse-switches. The price of the protection system is round 6 [3][4]. The 1 V distribution system requires new distribution transformers. Standard 2/.4 kv distribution transformers have been good base for developing 2/ distribution transformers. 1 V system requires also 1/.4 kv transformers to change the voltage level suitable for customers. In 1 V installations of SSS Ltd 1/.4 kv transformers are customized from similar 1/.4 kv transformers, which provides a good starting point for design. New series of 1/.4 kv transformers has been designed. Main specifications for the new 1/.4 kv transformers were low price small physical dimensions outdoor and indoor installable relatively small losses and maintenance costs dry-core construction suitability for unbalanced load for example Dyn-vector group Nominal powers of the designed transformers were 1 to 5 kva. Parameters of transformers are presented in table 1. TABLE 1 Parameters for 1/.4 kv distribution transformers. 1 kva 16 kva 25 kva 5 kva P [%].6.55.5.4 P k [%] 3.5 3. 2.5 2.2 k [%] 4.5 4.2 3.7 3.7 Price [ /pc.] 75 78 92 1715 Prognoses of the future mass production prices were calculated on the basis of the prices submitted by the manufacturer. The average unit price reduction was predicted to be at least 3 % in the mass production [4]. TECHNO-ECONOMIC ANALYSES The theoretical efficiency analysis consists of two phases. The first phase is to analyse the economical efficiency of the 1 V distribution system compared to the 2 kv medium voltage line. One of the main targets is to U 2 determine the range of use of the 1 V line as a function of distributed power, and the length of the line. The second target is to determine the economical efficiency of the 1 V system as a part of the low voltage network, and especially to determine in which cases it is economical to use the 1 V system and in which not. The research was done through theoretical network designs. The lined-up customer array was selected as the main topology for the theoretical designs. An overall guideline to find the most economical structure of the distribution network and to answer the question of what are the cases where the use of the 1 V distribution system is most economical can be found by combining the introduced analysis. In the calculations the unit costs presented in national cost list of network components (KA 2:23) [5] are used as the costs of the network components. The costs of the 1/.4 kv transformers are given in table 1. The costs of the needed distribution substation for 1/.4 kv pole transformer are approximately 7 and for cabin substation 125. Other used calculation parameters are presented in table 2. Unit costs for interruptions are presented in [6]. TABLE 2 Used calculation parameters. Parameter Value Lifetime [a] 4 Time of load growth [a] 4 Peak operating time of losses [h] 1 Interest rate [%/a] 5 Power factor.95 Annual growth of consumption [%/a] 1 Price of power losses [ /kw] 3 Price of energy losses [ /kwh].3 Comparison of 2 kv and lines The considered cost factors of the 1 V distribution system are the costs of conductors, transformers, substations, maintenance, interruptions and fault repairing. With the 1 V voltage level the power transmission rate is 6.25 times greater compared to same size of cable at 4 V. The total discounted lifetime maintenance costs for the medium voltage network are approximately 163 /km and for low voltage network 824 /km. The fault repair costs are correspondingly 1393 /km and 549 /km. The savings of the line path between the overhead line and overhead cable are included in the investment costs of conductors presented in KA 2:23. Interruption costs are defined for the medium voltage as the average interruption costs for an average rural medium voltage feeders and average urban medium voltage feeders for a one hour of interruption. The average discounted lifetime interruption costs are for rural feeder 15 /km and for urban feeder 223 /km. The interruption costs in the medium voltage network are assumed to be constant regardless the transferred power in the calculated branch. However, for the low voltage network the interruption costs are considered to be a
function of transferred power and type of customers. For the low voltage side all customers are supposed to be residential customers. The interruption time is one hour. The investment costs of 1 V line are lower compared to regular overhead medium voltage line. However, the 1 V system includes some extra costs, especially the investment costs of the 1/.4 kv transformers are remarkable. Costs of losses in 1 V line are higher compared to typical medium voltage overhead line. Costs of outages and costs of maintenance and fault repair are lower for 1 V line. Based on different cost components it is possible to calculate the minimum economical line length for the 1 V line. The voltage drop restricts the maximum length of the 1 V line. In following figures examples of economical usage areas of the 1 V system for an aerial brunched and an underground cable are presented. Length of line [km] Fig. 3 Length of line [km] 6 5 4 3 2 1 Fig. 4 6 5 4 3 2 1 Maximum length Minimum length for rural areas Minimum length for town areas Targets from a MV -feeder Economical area 5 15 25 35 45 55 65 75 85 95 15 115 125 135 Power [kw] Economical range of use of a 1 V aerial brunched cable (AMKA 7) compared to a 2 kv medium voltage overhead line. Maximum length Minimum length for rural areas Minimum length for town areas Targets from a MV -feeder Economical area 5 15 25 35 45 55 65 75 85 95 15 115 125 135 Power [kw] Economical range of use of a 1 V underground cable (AXMK 7) compared to a 2 kv medium voltage overhead line. In the Fig 3 and Fig. 4 combined maximum allowed voltage drop in the 1 V cable and 1/.4 kv transformer is 8 %. Medium voltage conductor is AF 4. It can be seen that the 1 V line is more economic compared to medium voltage line in rural areas where power is less than 8 kw and line length is few kilometres. In populated areas the economical minimum length of line is smaller than in rural areas because of higher interruption costs in medium voltage network. Then the 1 V line is economical when power of line is less than 11 kw. Between aerial brunched cable and underground cable there is no remarkable difference in economical areas compared to medium voltage overhead line. The economical efficiency of 1 V line compared to 2 kv medium voltage line depends on interruption costs. If interruption costs are smaller than presented, for example the interruption time shortens, the economical range of 1 V line reduces. In practise there are lot of potential installation targets for 1 V system. In Fig. 3 and Fig. 4 there are presented 2/.4 kv distribution substations of SSS Ltd that could be replaced with 1 V system. Comparison of traditional and three voltage level distribution To find out a situation where the 1 V system is economical, the traditional and three-voltage-level system have to be compared as a one. The comparison is done for different sizes of distribution substations in certain situations. The use of the 1 V distribution system is profitable when the costs of the traditional system are higher or equal to the costs of the 2/1/.4 kv system. In the Fig. 5 cost diagrams of 2/1/.4 kv system and 2/.4 kv system are presented when the customers are located at end of a 4 km branch line. The customer density is 1 customers/km. 5 % peak power of a customer is 5 kw. Total costs [k /customer] Fig. 5 1 9 8 7 6 5 4 3 2 1 a 2/1/.4 kv system 2/.4 kv system 5 1 15 2 25 3 35 4 45 5 Customers in distribution substation Costs of a 2/.4 kv system and a 2/1/.4 kv system when the customers are in the end of a 4 km branch line. In Fig. 5 the 1 V line replaces the medium voltage branch line needed in the traditional system when there is less than 7 customers. Between 8 to 26 customers (between a-b) 2/1/.4 kv system is not economical compared to traditional system because of the costs of the 1/.4 kv transformers. After 27 customers the traditional distribution system needs to be divided in to two 2/.4 kv distribution substations. With 2/1/.4 kv system over 5 customers can be fed with single 2/ transformer because of smaller losses of 1 V lines compared to 4 V lines. If there is no need for any medium voltage branch lines, for example in situation where customers are located near by the medium voltage mainline, the traditional system is the most economical solution. The theoretical network designs have shown that the three-voltage-level system is b
an economical solution compared to the traditional system when it makes it possible to replace medium voltage lines with 1 V line. As a part of actual low voltage distribution the 1 V system is economical when it enables feeding more customers with smaller amount of medium voltage to low voltage distribution substations compared to traditional system. The greatest advantage is then the better reliability of the 2/1/.4 kv system compared to the 2/.4 kv system. An interruption in the medium voltage network often affects hundreds of customers, but in the 1 V system it only affects the customers under the 2/1/.4 kv distribution substation. However, the total amount of distribution transformers is higher than in traditional system because of the needed 1/.4 kv transformers. The experiences of the 2/1/.4 kv system are positive and it has fulfilled most of its expectations. All the targets where the 1 V system has been used have worked properly and reliably. Especially the customers in the lake district of central Finland have been very satisfied. Because of the unnoticeable structure of the 1 V lines they fit better to the environment than typical medium voltage lines. The following map (Fig. 7) presents a 1 V line installed in the distribution network in SSS Ltd. 1/ PRACTICAL EXPERIENCES 1/ The 1 V distribution system has been a part of the distribution network of SSS Ltd for few years. The official introduction was in autumn 21. All the installations made by SSS Ltd have been done to avoid medium voltage branches. Since 21 the 1 V system has become a part of the normal network design in SSS Ltd. The 1 V system is today used in about 3 targets and the number of application areas is increasing rapidly when the company replaces over 4-year-old medium voltage lines in a sparsely populated area with the three-voltage-level system [3]. The introduction of the system was originally complicated by the lack of 1 V voltage classification for some low voltage components. The authorities were also cautious in trusting the calculations made by SSS Ltd because there were no precedents of building such a system in Finland. However, the component suppliers have been able to meet the demand of components fairly fast. Another challenge was renewing the network databases to support the 1 V for accurate calculations [3]. The 1 V system makes the network topology more complex than before. It also increases the amount of network components. Constructing these kinds of complex networks is in contradiction with the traditional principles of network design. However, the technical development has given many new possibilities to construct more reliable networks even if they are complex. The 1 V system offers many new possibilities to develop the distribution. With the 1 V system the branches of the main lines can be separated to independent protection areas and so faults on the branches do not interrupt the whole distribution. However, this increases the need of operation control in the distribution. The savings and technical advantages achieved with 1 V system can be used to improve the medium voltage network. For example by increasing the amount of underground cables in main lines. 1 km Fig. 6 1 V line at Kongonsaari, Finland [3]. 2/ 2 kv The first 1 V line was built in area of Kongonsaari. It was the test installation for the protection components and 1/.4 kv transformers. Kongonsaari is located south of Savonlinna, and the main advantage of the 1 V system is protecting the lake environment. The 1 V system also diminished faults in the area because the medium voltage overhead line would have been built in the forest. CONCLUSIONS The 1 V distribution system is economical as a replacement of a 2 kv medium voltage line in the power range of 1 kw and in line lengths starting from 15 m with the introduced calculation parameters. The economical range of the 1 V line length is a function of used components, costs and power and is restricted by technical boundaries. Another application area of the 1 V system is as a part of a low voltage network as a replacement of a long.4 kv line for example in a lake district. The benefits of the 1 V system in these targets are lower investment costs than with a medium voltage line and a higher transmission rate than with a.4 kv low voltage line.
REFERENCES [1] European Comission, 1973, LVD Low voltage directive 73/23/EEC 19.2.1973. [2] Lakervi, E., Holmes, E.J., 1995, Electricity Distribution Network Design, 2 nd edition. Peter Peregrinus Ltd., Reprinted 1998. Short Run Press Ltd., Exeter England. ISBN 86341 39 9. 192-28 [3] Lohjala, J., Suur-Savon Sähkö Ltd. Kaipia, T., Lassila, J., Partanen, J., Lappeenranta University of Technology, 24, Overview to economical efficiency of 1 V low voltage distribution systems. Proceedings of the NORDAC 24 Conference, Espoo, Finland, Augustn 24. 18 p. [4] Kaipia, T., 24, Economic efficiency of 1 V electricity distribution system. Master thesis. Lappeenranta University of Technology, Lappeenranta, Finland 24. [5] Finnish Electricity Association, SENER ry, 24, KA 2:23 National cost list of network components. Adato Energia Ltd., Helsinki, Finland 24. [6] Kivikko K., Mäkinen A., Verho P., Järventausta P., Tampere University of Technology. Lassila J., Viljainen S., Honkapuro S., Partanen J, 24. Outage Cost Modeling for Reliability Based Network Planning and Regulation of Distribution companies. Proceedings of the DPSP 24 Conference, Amsterdam, Netherlands, April 24. 4 p.