Overview Kaipia 1000 v Distribution

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OVERVIEW TO ECONOMICAL EFFICIENCY OF 1000 V LOW VOLTAGE DISTRIBUTION SYSTEMS

Juha Lohjala. Suur-Savon Sähkö Ltd. Tero Kaipia, Jukka Lassila, Jarmo Partanen

Lappeenranta University of Technology

Contact information: [email protected]

[email protected] [email protected]

[email protected]

INTRODUCTION

The need to improve the quality and economy of the electricity distribution process has increased year by year. One very interesting new innovation in the distribution network development has been the use of 1000 V low voltage lines together with 20 kV and 0.4 kV systems. The EU-legislation enables the use of 1000 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. This affects the interruption costs of the entire distribution network.

This paper presents the main principle of the new 20/1/0.4 kV system, techno-economical analyses of the usability of the three-stage system as well as some case examples of the three-stage distribution system already used in Suur-Savon Sähkö distribution company. The economic efficiency is considered by comparing the costs of the traditional two-stage and the three-stage distribution network defined here. Both network solutions are optimised with regard to the costs and technical boundaries. In the calculations the distribution network is defined through theoretical network designs that are made for some basic network topologies. The cases of the use of 1000 V distribution voltage concern some experiences gained from the network of Suur-Savon Sähkö Ltd. The three-voltage-level distribution system has been in use for several years in lake district. The experiences are very promising.

TECHNICAL DESCRIPTION OF THE 1000 V DISTRIBUTION SYSTEM

To improve the quality of the distribution economically, new kinds of network solutions are needed. Because the investments made in the distribution network are usually high, the solutions have to be considered carefully. Long term investments that during the lifetime of the network reduce its operation and maintenance costs and improve the quality of the delivered electricity are needed. These kinds of investments aim at shortening the medium voltage branches of the distribution network and changing over to cables with better immunity to outer disturbances.

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 50 to 1000 V and for direct voltage from 75 to 1500 V. According to the directive, instruments that are

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classified for these voltage ranges are low voltage instruments. This makes it possible to use the 1000 V voltage level in the low voltage distribution network.

The 1000 V low voltage level is used between the 20 kV medium voltage network and 0.4 kV low voltage network. Figure 1 is presents example network topologies for feeding few resident customers with traditional two-stage and with three-stage distribution. The situation is fictitious.

a) b)

20 kV

Z

Z

0,4 kV

0,4 kV

20 kV

Z

Z

1 kV

0,4 kV

1 kV

0,4 kV

0,4 kV

a) b)

20 kV

Z

Z

0,4 kV

0,4 kV

20 kV

Z

Z

1 kV

0,4 kV

1 kV

0,4 kV

0,4 kV

Figure 1 Feeding residential customers with a a) traditional and b) three-stage

distribution solution.

As presented in figure 1, using the 1000 V voltage level makes it possible to reduce the length and number of branches in the medium voltage network. This, especially in the overhead line network, diminishes the possibility of blackouts for the entire medium voltage line and so affects the interruption costs and the quality of distribution.

Operation of the 1000 V distribution system

Based to theoretical examinations and measurement results from the experimental installations of Suur-Savon Sähkö Ltd (later SSS Ltd) the 1000 V part of the distribution network is operated as isolated from ground. Usually the low voltage network is operated as grounded. This means that the system has a zero wire that is connected to ground and to the star point of the distribution transformer. However, the safety regulations define that the voltage between the ground level and the zero wire of the system can not exceed 75 V in any part of the low voltage network during any possible fault situation (Lakervi 1998). In common Finnish grounding circumstances this rule is almost impossible to fulfil with the 1000 V system, which has been demonstrated in the measurements done by SSS Ltd. The 0.4 kV low voltage network starting from 1/0.4 distribution transformers is operated as grounded.

Another reason for operating the 1000 V system isolated from ground is the reliability of protection. If the 1000 V voltage level could be operated as a grounded network, the protection would be easy and inexpensive to carry out with fuses as in the 0.4 kV network. However, the protection would then bind the length of 1000 V lines as in the 400 V network. Also the reliability of the protection would be bad for example in the situation where the zero-conductor is cut between the fault location and 20/1 kV distribution transformer station. Then there will also be exceeding of the 75 V touch up voltage at the customer’s end of the low voltage network. When the 1000 V network is operated as isolated from ground the protection is executed with relays and circuit breakers. Then the only limit for the length of the 1000 V

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line comes from the voltage drop of the used cables. Figure 2 shows the principle of the 1000 V low voltage network.

Consumer

Figure 2 The principle of operating the 1000 V low voltage network.

Protection of the 1000 V distribution system

As mentioned abowe, the protection of the 1000 V low voltage distribution system is done with relays and circuit breakers. 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 1000 V network, the direction of the fault current does not have to be known. So the earth fault can be ascertained for example with

• measuring the possible earth fault current, • measuring the open triangle voltage, • measuring the potential between the star point and ground of the system.

In SSS Ltd the method used is the measurement of the potential between the star point of the 20/1 kV transformers and ground. The choice was made on the basis of the costs of the protection system. The measurement of the ground potential needs only a few components and is simple. Only one voltage transformer between star point and ground potential is needed. The principle of earth fault protection is presented in figure 3.

L1

L2

L3 M K1 K2

U2U1

L1

L2

L3 M K1 K2

U2U1

Figure 3 Earth fault protection of the 1000 V distribution system.

The components of the earth fault protection system presented in figure 3 are

M = voltage transformer (Sn = 140 VA, U1/U2 = 575/230 V) K1 = time lock K2 = trip relay

Time lock relay is needed to prevent false operation of the protection in short time earth faults like for example operation of an arrester during a thunder storm.

The complex protection system is more expensive than normal fuse protection. However, the advantage of the system is that it does not restrict the length of the 1000 V line like fuse protection. In practise the used protection system is integrated in one package and can be

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installed for example to a pole like pole-fuse-switches. The price of the protection system is round 600 €.

The above kind of protection system is used in the installations of SSS Ltd. The system has worked properly so far. However, the final reliability of the protection system will be tested with time. Another prospect for the future is that this kind of protection devices will enable cost effective expansion for the range of use of the distribution network automation. This will take the control over the distribution network nearer the customer than traditionally.

Distribution transformers and substations

Standard 20/0.4 kV distribution transformers are a good base to develop 20/1 kV distribution transformers. Distribution substations for 20/1 kV transformers can be exactly the same as with 20/0.4 transformers. The only difference is that with 20/1 kV transformers the 0.4 kV fuse-switches have to be changed to a 1 kV protection package.

The insulation level of the transformer determines the higher voltage used in the transformer. In 20/1 and 20/0.4 kV transformers it is the same, so the insulation level in both these transformer types is the same. In theory also the dimensions of the transformers’ cores are the same, because they are determined by the power of the transformer. So it can be simplified that the only difference between 20/0.4 kV and 20/1 kV distribution transformers is in their windings. The costs of the 20/1 kV transformers are nearly the same as the costs of the 20/0.4 kV transformers.

The 1000 V distribution system requires also a second type of distribution transformer. 1/0.4 kV transformers are needed to change the voltage level suitable for customers. For these kinds of distribution transformers there is no example in the traditional distribution network. In 1000 V installations of SSS Ltd 1/0.4 kV transformers are custom made in same format as the 20/0.4 kV transformers, which is a good starting point for design. A new series of 1/0.4 kV transformers was designed with the help of a transformer manufacturer. The specifications for the new 1/0.4 kV transformers are

• low price • small physical dimensions • outdoor and indoor installable • relatively small losses and maintenance costs

o dry-core transformers • capability to handle unbalanced load

o for example Dyn-vector group

The transformers were designed for the nominal powers of 10 to 50 kVA. Some technical parameters of the designed transformers are presented in table 1 below.

Table 1 Technical parameters for 1/0.4 kV distribution transformers. 10 kVA 16 kVA 25 kVA 50 kVA

P0 [%] 0.60 0.55 0.50 0.40 Pk [%] 3.5 3.0 2.5 2.2 Zk [%] 4.5 4.2 3.7 3.7

Efficiency [%] 95.9 96.5 97.0 97.4 Weight [kg] 90 120 170 290

Standard case [mm] 570x450x580 570x450x580 570x450x580 740x500x800 Small case [mm] 510x360x530 510x390x530 570x430x580 680x450x750

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Some prognoses of the future mass production prices were calculated on the basis of the prices submitted by the manufacturer. In the mass production the average drop of price was predicted to be at least 30 % of the current unit cost. The unit costs and predicted mass production costs of the 1/0.4 kV transformers introduced in table 1 are presented in table 2.

Table 2 Unit prices of the 1/0.4 kV transformers. Current unit price [€/pc.]

Nominal effect [kVA] For one piece For 5 pieces For 25 pieces Mass production unit price [€/pc.]

10 705 670 620 491 16 778 739 685 542 25 918 872 808 640 50 1715 1629 1509 1195

ECONOMICAL EFFICIENCY ON THE BASIS OF THEORETICAL ANALYSIS

The theoretical efficiency analysis consists of two phases. The first phase analyses the economical efficiency of the 1000 V distribution system compared to the 20 kV medium voltage line. One of the main targets is to determine the range of use of the 1000 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 1000 V system as a part of the low voltage network, and especially to determine in which cases it is economical to use the 1000 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 the cases are where the use of the 1000 V distribution system is most economical, can be found by combining the introduced analysis. The unit costs presented in cost list KA 2:2003 (KA2:2003) are used as the costs of the network components, except for the 1/0.4 kV transformers. The costs of the 1/0.4 kV transformers are given in table 2. The costs of the needed distribution substation for pole transformer are approximately 700 €. The other calculation parameters used in analysis are presented in table 3.

Table 3 Used calculation parameters. Parameter Value

Lifetime [a] 30 Time of load growth [a] 10 Peak operating time of losses [h] 1000 Interest rate [%/a] 5 Power factor 0.95 Annual growth of consumption [%/a] 3 Price of power losses [€/kW] 30 Price of energy losses [€/kWh] 0.03

Comparison of 20 kV and 1 kV lines

The considered cost factors of the 1000 V distribution system are the costs of the conductors, transformers, substations, maintenance, interruption and fault repairing. The starting point for the first comparison is presented in figure 4. With the 1000 V voltage level the power

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transmission rate is 6.25 times better than with same kind of a cable and 400 V voltage. 20 times more power can be transferred 20 times further with the 20 kV line compared to a 1000 V line, and 50 times more power 50 times further compared a 400 V line. To compare different voltage levels it is necessary to take both cost factors and technical factors into account.

Z

20/ 0,4 kV

The 20/ 0,4 kV systemZ

0,4 kV20 kV

L

20/ 1 kV

1/0,4 kV

ZZ

1 kV20 kV 0,4 kV

L

The 20/ 1/0,4 kV system

Z

20/ 0,4 kV

The 20/ 0,4 kV systemZ

0,4 kV20 kV

L

20/ 1 kV

1/0,4 kV

ZZ

1 kV20 kV 0,4 kV

L

20/ 1 kV

1/0,4 kV

ZZ

1 kV20 kV 0,4 kV

L

The 20/ 1/0,4 kV system

Figure 4 Comparison of a 1000 V line and a 20 kV line.

The first comparison is made by a function of transferred power for a one-kilometre long line. With the calculation parameters presented in table 3, the maintenance costs for the medium voltage network are approximately 1460 €/km and for low voltage network 740 €/km. The fault repair costs are correspondingly 1240 €/km and 242 €/km. These costs are total discounted costs of the whole lifetime (30 a). The savings of the line street between the overhead line and overhead cable are included in the investment costs of conductors presented in KA 2:2003.

Interruption costs are defined for the medium voltage side as the average interruption costs for an average medium voltage feeder. For an average medium voltage overhead line feeder in Finland the power is 900 kW, the number of customers is 674 and the length of line 35 km (EMV 2002)(Sener 2002). The division of consumption in Finland and the interruption costs for each customer group are presented in table 4. The consumption on the average medium voltage feeder is assumed to be divided similarly. When the interruption time is one hour, the interruption costs are divided as shown in table 5.

Table 4 Interruption costs per customer group (EMV 1/2003) and their proportion of electricity consumption in Finland (Adato 2002).

Interruption costs for fault situation

Customer group €/kW €/kWh Proportion electricity consumption Residential 0.068 0.61 41 % Agricultural 0.54 4.9 6 %

Industry 2.60 8.7 21 % Public 0.65 3.4 11 % Service 1.90 11.0 21 %

Table 5 Interruption costs for the average medium voltage feeder. Customer group Average power [kW] Interruption costs [€/h]

Residential 368 249 Agricultural 54 291

Industry 188 2122 Public 100 405 Service 192 2482 Total 901 5549

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According to the statistics, the average interruption frequency in the medium voltage overhead line network is 4.92 faults/km,a (Sener 2002). On the basis of this the interruption costs for the average medium voltage feeder for every added kilometre of new line is 273 €/km,a for a one hour interruption time. Now for example with the lifetime of 30 a and with 1% annual growth of consumption in the medium voltage network, the interruption costs discounted to the present are 4744 €/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 side of the system 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 and the interruption time one hour.

A comparison can now be done by combining the following costs of one kilometre line independently for each voltage level. The combined costs per kilometre are presented in figure 5.

• investments costs o 20 kV line o 1 kV line o 1/0.4 kV distribution substations and transformers

• interruption costs • fault repair costs • maintenance costs • losses

o conductors o 1/0.4 kV transformers

The 1000 V system has lower conductor costs than the medium voltage network, but it needs a 1/0.4 kV transformer. However, the 1000 V system diminishes the interruption costs in the medium voltage network because the faults in the low voltage side of the branch do not affect the whole medium voltage feeder.

10000

15000

20000

25000

30000

0 10 20 30 40 50 60 70 80

Siirtoteho [kW]

Kus

tann

ukse

t [€/

km]

Raven

Sparrow

AMKA 35AMKA 120

Power [kW]

Cos

ts [

€]

Figure 5 Costs of a 1000 V line and a medium voltage overhead line as a function of

transferred power when L = 1km. The bumps in the graphs of AMKA cables are caused by the costs of the 1/0.4 kV transformer change.

The 1000 V system is cheaper than the regular overhead medium voltage line. However, the economic power area depends greatly on the cost factors, especially on the costs of interruption. With the presented calculation parameters a 1 km long 1000 V line is more economical than a medium voltage overhead line in the power range from 0 to 65 kW.

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Because the 1000 V line needs extra transformers etc., it has cost factors which make the system uneconomical, compared to the medium voltage line in zero length of the line. The savings achieved with the 1000 V system compared to the 20 kV overhead line are presented in figure 6. As the figure shows, compared to the medium voltage line there is a line length in which the 1000 V system is uneconomical. In the other end the greatest economical length of the 1000 V line is restricted by the voltage drop. The comparison medium voltage cable is Raven.

-100000

100002000030000400005000060000700008000090000

100000

0 1 2 3 4 5 6 7 8

Length of line[km]

Spar

e [€

]

AMKA120AMKA35

Savi

ngs [

€]

Figure 6 Achieved savings of a 1000 V AMKA –line compared to a 20 kV Raven -

overhead line when the transferred power is 30 kW.

The vertical lines in figure 6 represent the maximum length of 1000 V line when the allowed voltage drop in the line and 1/0.4 kV transformer is 6 %. The negative savings at zero length comes from the costs of a 1/0.4 kV distribution substation and transformer and is a function of transferred power. However, the system also has cost factors that are only a function of line length, such as maintenance and fault repair costs, which seem to have a great influence on the economical range of use of the 1000 V line as a replacement of the medium voltage overhead line.

One interesting question is the influence of the investment costs of the 1/0.4 kV transformers on the economical efficiency of the 1000 V line. The effect of the mass production costs (-30 %) of the 1/0.4 kV transformers on the achieved savings of the 1000 V line compared to the medium voltage overhead line is presented in figure 7.

-6000

-4000

-2000

0

2000

4000

6000

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1

Johdon pituus [km]

Sääs

töt [

€] nykykustannukset

muuntajakustannukset -30 %

Length of line [km]

Spar

e [€

]

Current prices

Mass production prices

Savi

ngs [

€]

Figure 7 The effect of the investment costs of 1/0.4 kV transformers on the achieved

savings of an 1000 V AMKA –line compared to a medium voltage overhead line.

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A situation where a 4 km branch is needed to transfer 30 kW power to residential customers at the end of the line can be considered as an example. If the maximum allowed voltage drop in this line is 6 % the whole branch can be built with an AMKA 120 cable and a 1000 V system. Correspondingly, with the traditional system at least 3 km of the branch must be constructed as a medium voltage line to achieve an economical state. When these two solutions are compared, it can be seen that the most economical solution is to use the 20/1/0.4 kV system.

The range of use for the 1000 V system can be defined by combining the two introduced analysis of comparing 20 kV and 1 kV lines. This kind of analysis is necessary. However, the two graphs shown in figure 8 are true only for cases where one 1/0.4 kV transformer in the end of the branch is needed. An individual analysis has to be done for every situation. Figure 8 shows the minimum economical length of the 1000 V line compared to the 20 kV line, and the maximum length of line when the combined maximum voltage drop in the 1000 V cable and 1/0.4 kV transformer is 6 %. The conductors used in the comparison are Raven for the medium voltage line and AMKA 120 for the 1000 V line.

02000400060008000

100001200014000160001800020000

10 20 30 40 50 60 70 80 90 100

Power [kW]

Leng

th o

f lin

e [m

]

Maximum length of line Minimum economical length of line

Economical range

Figure 8 Economical range of line length for a 1000 V system compared to a 20 kV

medium voltage overhead line.

Adding more 1/0.4 kV transformers to the end of the branch narrows the economical range of the use of the 1000 V system between the lines in figure 8. The maximum length of 1000 V line is restricted by technical boundaries and the minimum length by the costs of the 1/0.4 kV distribution transformer and the substation. The maximum capacity of the 1000 V AMKA 120 cable is 411 kW with power factor 0.95.

Economical number of customers connected to a 1/0.4 kV transformer

To apply the 1000 V distribution system economically as a part of a low voltage network it is necessary to solve the economical number of customers under a 1/0.4 kV distribution substation. This number is a function of power and type of customers as well as the customer density connected to the substation. A correlation between the economical number of customers connected to a 1/0.4 kV transformer and the relation of the costs of the 1/0.4 kV transformer station and the needed 0.4 kV network, can be found. Figure 9 contains a calculated cost diagram in a situation where the distance between the residential customers is 0.1 km and the average consumption of a customer is 5, 10 and 15 kW.

10

0

500

1000

1500

2000

2500

3000

3500

4000

1 2 3 4 5 6 7 8 9 10

1/0,4 muuntopiirin asiakasmäärä

kust

annu

kset

[€/a

siak

as]

5 kW 10 kW 15 kW

Customers in 1/0,4 kV distribution substation

Cos

ts [k

€/cu

stom

er]

Figure 9 Costs of 1/0.4 kV distribution per customer as a function of the number of

customers under the 1/0.4 kV distribution substation.

As figure 9 shows, the economical number of customers under the 1/0.4 kV substation is quite wide. An increase in the average consumption of a customer narrows the economical range. The cost factors influence the economical number of customers. For example figure 11 presents the effect of 1/0.4 kV investment costs to the economical number of customers as a function of customer density in a 0.4 kV network.

0

1

2

3

4

5

6

7

8

9

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

asiakastiheys [1/km]

edul

lisin

1/0

,4 k

V m

uunt

amo

asia

kasm

äärä

Density of residential customers [1/km]

Cus

tom

ers i

n 1/

0,4

kV

dist

ribut

ion

subs

tatio

n

Current prices Mass production prices

Figure 10 Economical number of customers in a 1/0.4 kV distribution substation as a

function of customer density of the distribution substation with current 1/0.4 kV transformer prices and mass production prices.

When the customer density decreases, smaller substations become economical. The 1000 V network affects the economical number of customers connected to an individual 1/0.4 kV transformer. If the 1/0.4 kV substation is in the border area of the 1000 V network and the distance from the 20/1 kV substation is long, smaller number of customers in the current 0.4 kV network becomes economical. This is mainly an effect of technical boundaries. Figure 11 shows the relationship between the number of customers in a 1/0.4 kV substation and total number of customers in a 20/1/0.4 substation when the customer density is 10 customers/km and the average consumption of a customer 5 kW.

11

0123456789

1011121314

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75

asiakasmäärä

1/0,

4 kV

muu

ntam

on a

siak

asm

äär

Customers in 20/1/0,4 kV distribution substation

Cus

tom

ers i

n 1/

0,4

kV

dist

ribut

ion

subs

tatio

n

0123456789

1011121314

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75

asiakasmäärä

1/0,

4 kV

muu

ntam

on a

siak

asm

äär

Customers in 20/1/0,4 kV distribution substation

Cus

tom

ers i

n 1/

0,4

kV

dist

ribut

ion

subs

tatio

n

Figure 11 Economical number of customers in a 1/0.4 kV distribution substation as a

function of the number of customers in a 20/1/0.4 kV distribution substation.

The interest rate has only a small effect on the economical number of customers connected to a 1/0.4 kV substation compared to the dispersion of the load curve. If the dispersion is vast, it also evens out quickly compared to average power when the number of customers increases. This makes the economical range of the number of customers wide. However, these calculation results are valid only in the situation when the 1000 V system is economical as a part of the low voltage network.

Comparison between traditional and three voltage level distribution

To find out a situation where the 1000 V system is economical, the traditional and three-voltage-level system have to be compared as a whole. The comparison is done for different sizes of distribution substations in certain situations. The use of the 1000 V distribution system is profitable when the costs of the traditional system are higher than or equal to the costs of the 20/1/0.4 kV system. In the following examinations the customer density is 10 customers/km and the average consumption of a residential customer 5 kW.

Cost diagrams for both systems when the customers are in the vicinity of the 20 kV medium voltage line and there is no need of a branch line are presented in figure 12. As can be seen, the traditional system is more economical in this situation in the whole range.

00,5

11,5

22,5

33,5

44,5

5 10 15 20 25 30 35 40 45 50 55 60

Asiakkaiden määrä

Kus

tann

ukse

t [k€

/asi

akas

]

20/1/0,4 kV muuntopiirin asiakasmäärä Customers in 20/1/0,4 kV distribution substation

2-voltage level system

3-voltage level system

Cos

ts [k

€/cu

stom

er]

Figure 12 Costs of a 20/0.4 kV system and a 20/1/0.4 kV system when the customers are in the vicinity of a 20 kV medium voltage line.

What is the situation when a branch line is needed? In the following three figures some example designs to feed different numbers of lined-up customers are presented. Figure 13

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shows an example of feeding a customer group of 5, figure 14 a customer group of 20 and figure 15 a customer group of 40. The length of the needed branch is 4 km. Also the costs of these kinds of network solutions are presented in the figures.

Z ZZ

20 kV

1/0,4 kV

5 customers 5 customers

20/1 kV

Total 82 k€ Total 95 k€Uh max ~ 8 % Uh max ~ 9 %

a) b)

Investments:20/ 1 kV transformer 6139 €1 kV line 64640 €1/0,4 kV transformer 2415 €0,4 kV network 3456 €Total 76650 €Losses:20/ 1 kV transformer 1223 €1 kV line 1121 €1/0,4 kV transformer 1637 €0,4 kV network 1389 €Total 5370 €Interruption:1 kV line 29 €

0 km

4 km

Z Z

Z

20 kV

3,2

km0,

8 km

Investments:20 kV line 54174 €0,4 kV line 12436 €20/ 0,4 kV transformer 6139 €0,4 kV network 3456 €Total 76206 €Losses:20 kV line 19 €0,4 kV line 1332 €20/ 0,4 kV transformer 1223 €0,4 kV network 1389 €Total 3963 €Interruption:20 kV line 15325 €0,4 kV line 6 €Total 15331 €

250 m250 m

20/0,4 kV

250 m250 m

Z ZZ

20 kV

1/0,4 kV

5 customers5 customers 5 customers5 customers

20/1 kV


a) b)

Investments:20/ 1 kV transformer 6139 €1 kV line 64640 €1/0,4 kV transformer 2415 €0,4 kV network 3456 €Total 76650 €Losses:20/ 1 kV transformer 1223 €1 kV line 1121 €1/0,4 kV transformer 1637 €0,4 kV network 1389 €Total 5370 €Interruption:1 kV line 29 €

0 km

4 km

Z Z

Z

20 kV

3,2

km0,

8 km

Investments:20 kV line 54174 €0,4 kV line 12436 €20/ 0,4 kV transformer 6139 €0,4 kV network 3456 €Total 76206 €Losses:20 kV line 19 €0,4 kV line 1332 €20/ 0,4 kV transformer 1223 €0,4 kV network 1389 €Total 3963 €Interruption:20 kV line 15325 €0,4 kV line 6 €Total 15331 €

250 m250 m

20/0,4 kV

250 m250 m

Figure 13 Example network for feeding five line- up customers a) with a three-voltage-level

system and b) with a traditional system.

Z Z

Z

20 kV

1/0,4 kV


20/1 kV


a) b)

Investments:20 kV line 66642 €20/ 1 kV transformer 7592 €1 kV line 422 €1/0,4 kV transformer 6286 €0,4 kV network 32260 €Total 113202 €Losses:20 kV line 151 €20/ 1 kV transformer 3104 €1 kV line 115 €1/0,4 kV transformer 4761 €0,4 kV network 3786 €Total 11917 €Interruption:20 kV line 18852 €1 kV line 1 €Total 18853 €

3,97

km

0,03

km

Z Z

Z

20 kV

4 km

0 km

Investments:20 kV line 67080 €20/ 0,4 kV transformer 7592 €0,4 kV network 32260 €Total 106932 €Losses:20 kV line 152 €20/ 0,4 kV transformer 3104 €0,4 kV network 3786 €Total 7042 €Interruption:20 kV line 18976 €

1 km1 km

20/0,4 kV

1 km1 km

Z Z

Z

20 kV

1/0,4 kV


20/1 kV


a) b)

Investments:20 kV line 66642 €20/ 1 kV transformer 7592 €1 kV line 422 €1/0,4 kV transformer 6286 €0,4 kV network 32260 €Total 113202 €Losses:20 kV line 151 €20/ 1 kV transformer 3104 €1 kV line 115 €1/0,4 kV transformer 4761 €0,4 kV network 3786 €Total 11917 €Interruption:20 kV line 18852 €1 kV line 1 €Total 18853 €

3,97

km

0,03

km

Z Z

Z

20 kV

4 km

0 km

Investments:20 kV line 67080 €20/ 0,4 kV transformer 7592 €0,4 kV network 32260 €Total 106932 €Losses:20 kV line 152 €20/ 0,4 kV transformer 3104 €0,4 kV network 3786 €Total 7042 €Interruption:20 kV line 18976 €

1 km1 km

20/0,4 kV

1 km1 km

Figure 14 Example network for feeding 20 lined-up customers a) with a three-voltage-level


13

Z Z

Z

20 kV

1/0,4 kV


20/1 kV


a) b)

Investments:20 kV line 67080 €20/ 1 kV transformer 11439 €1 kV network 21434 €1/0,4 kV transformers (10 pcs .) 16280 €0,4 kV network 34560 €Total 150793 €Losses:20 kV line 563 €20/ 1 kV transformer 5168 €1 kV network 6788 €1/0,4 kV transformers (10 kpl) 16910 €0,4 kV network 1424 €Total 30853 €Interruption:20 kV line 18976 €

4 km

0 km

2 km2 km

Z Z

Z

20 kV

4 km

0 km

Investments :20 kV lines 80496 €20/ 0,4 kV transformers 14447 €0,4 kV network 64182 €Total 159125 €Losses:20 kV lines 654 €20/ 0,4 kV transformers 6912 €0,4 kV network 7683 €Total 15249 €Interruption:20 kV lines 28464 €

0,9 km0,9 km

20/0,4 kV18 as.

0 km 1,1 km1,1 km

20/0,4 kV22 as.Z 2 km Z

Z Z

Z

20 kV

1/0,4 kV


20/1 kV


a) b)

Investments:20 kV line 67080 €20/ 1 kV transformer 11439 €1 kV network 21434 €1/0,4 kV transformers (10 pcs .) 16280 €0,4 kV network 34560 €Total 150793 €Losses:20 kV line 563 €20/ 1 kV transformer 5168 €1 kV network 6788 €1/0,4 kV transformers (10 kpl) 16910 €0,4 kV network 1424 €Total 30853 €Interruption:20 kV line 18976 €

4 km

0 km

2 km2 km

Z Z

Z

20 kV

4 km

0 km

Investments :20 kV lines 80496 €20/ 0,4 kV transformers 14447 €0,4 kV network 64182 €Total 159125 €Losses:20 kV lines 654 €20/ 0,4 kV transformers 6912 €0,4 kV network 7683 €Total 15249 €Interruption:20 kV lines 28464 €

0,9 km0,9 km

20/0,4 kV18 as.

0 km 1,1 km1,1 km

20/0,4 kV22 as.Z 2 km Z

Figure 15 Example network for feeding 40 lined-up customers a) with a three-voltage-level


The example designs show that the three-voltage-level system is an economical solution compared to the traditional system when it makes it possible to replace the medium voltage line needed in the traditional system with 1000 V line. In low voltage distribution the 1000 V system has no use in situations shown in figures 13 and 14. The higher costs of the three-voltage-level system compared to the costs of the traditional system in some situations come mainly from the costs of the 1/0.4 kV distribution substations and from the extra length of required low voltage line. The change of the investment costs of the 1/0.4 kV transformers to the mass production costs have only a less than 3 % effect on the total costs of the substations in each presented situation.

In most cases the 1000 V distribution system is not economical as an actual part of the low voltage network. However, the system is an economical option in sparsely populated areas, where it can be used as the main line in the network. In most of these cases the economical efficiency of the use of the 1000 V system comes directly from the compensation of the need of a medium voltage line or a long 0.4 kV line. The greatest advantage is then the better reliability achieved with the 1000 V system compared to a medium voltage overhead line. An interruption in the medium voltage network often affects hundreds of customers, but in the 1000 V system it only affects the customers under the 20/1/0.4 kV distribution substation.

Another view is that by using a 1000 V system it is possible to build larger low voltage networks. This diminishes the need of distribution substations in the border of medium and low voltage network and the number of short branches in the medium voltage network. However, the total amount of distribution transformers is higher because of the needed 1/0.4 kV transformers. This is why the low voltage transformers have to be cheap and quite maintenance free.

PRACTICAL EXPERIENCES

The 1000 V distribution system has been a part of the distribution network of SSS Ltd for few years. The official introduction was in autumn 2001. All the installations made by SSS Ltd have been done to avoid medium voltage branches. Since 2001 the 1000 V system has

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become a part of the normal network design in SSS Ltd. The 1000 V system is today in use in 15 targets and the number of application areas will increase rapidly when the company replaces over 40-year-old medium voltage lines in a sparsely populated area with the three-voltage-level system.

The introduction of the system was originally complicated by the absent of 1000 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. However, the component suppliers have been able to meet the demand of components fairly fast. Another challenge was renewing the network data systems to support the 1000 V distribution voltage. It is necessary to involve the 1000 V system to network databases to make accurate calculations. SSS Ltd has also developed a marking system for the 1000 V system. Because the cables used in the 1000 V system are of same type as used in the 400 V systems there is a danger for a mix-up. That is why a 1000 V line is marked with triangle-shaped 1000 V –signs and yellow stripes round the poles.

The 1000 V system makes the network topology more complex than before. It also increases the amount of network components and cables. The new voltage level creates a new protection area, which increases the need of operation control in the distribution. Constructing these kinds of complex networks is in contradiction with the traditional principle of network design. The technical development gives many new possibilities to construct more reliable networks even if they are complex. The 1000 V system has diminished the need of building medium voltage overhead lines, which has improved the quality of the distributed electricity.

The 1000 V system offers many new possibilities to develop the distribution and the distribution network design. With the 1000 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. Because the aerial cables used in the low voltage network stand surface contacts without faulting, the maintenance costs of these cables are much smaller than with overhead lines. The savings of this can be used to improve the main medium voltage lines. When the network length of the medium voltage network shortens, the earth fault currents become smaller. Combining the achieved savings and small earth fault currents, one possibility to develop the medium voltage main lines is to increase the amount of underground cables in the medium voltage network.

The first prototypes of the 1000 V system have been well tried and the system is ready to be adapted to new areas. The experiences of the system are very positive and it has fulfilled most of its expectations. With the 1000 V system it is possible to improve the quality of distribution in sparsely populated areas economically with a small number of new network components. The main results of the research will be presented in the doctoral thesis of Juha Lohjala, the planning manager of Suur-Savon Sähkö Ltd.

Some examples of the 1000 V distribution system in use

All the targets were the 1000 V system has been used have been working properly and reliably. Especially the customers in the lake district of Central Finland have been very satisfied. Because of the very unnoticeable structure of the 1000 V lines they fit better to the delicate nature of the lake area than typical medium voltage lines. Also in other forest areas the 1000 V system saves the forest nature because the cables do not need a line gap. All the 1000 V systems constructed so far have been more economical than traditional 20/0.4 kV

15

systems. The following maps (figures 16 –18) show some of the installed 1000 V lines in the distribution network of SSS Ltd.

Figure 16 1000 V line at Kongonsaari.

The area of Kongonsaari was the first built 1000 V target. It was the test installation for the protection components and 1/0.4 kV transformers used by SSS Ltd. Kongonsaari is located south of Savonlinna, and the main advantage of the 1000 V system is protecting the delicate lake environment. The 1000 V system also diminished faults in the area because the medium voltage overhead line would have been built in the forest.

20/1 kV

1/0.4 kV

1/0.4 kV

16

Figure 17 1000 V line at Iso-Päistäre, Hirvensalmi.

Figure 18 1000 V line at Iikansaari,Hirvensalmi, Puulavesi

At Iikansaari there are five customers located on an island. The main advantage of using the 1000 V line was replacing the old 400 V underwater cable and adding the capacity of the line. This has made it possible to feed the recreational habitation on the island as well. At Iso-

20/1 kV

20/1 kV

1/0.4 kV

1/0.4 kV

17

Päistäre there were 6 new customers. The feeding of the recreational habitation on the island has been executed with a 1000 V underwater cable. If the feeding had been done with the traditional 20/0.4 kV system the 20 kV line would have been built through the forest on the other side of the island. That kind of a system is considerably more expensive than the used 1000 V line.

CONCLUSIONS

The 1000 V distribution system is economical as a replacement of a 20 kV medium voltage line in the power range of approximately 0 – 100 kW and in line lengths starting from approximately 150 m with the introduced calculation parameters. The economical range of the 1000V line length is a function of used cables, costs and power and is restricted by technical boundaries. Another application area of the 1000 V system is as a part of a low voltage network as a replacement of a long 0.4 kV line for example in a lake district. The benefits of the 1000 V system in these targets are lower investment costs than with a medium voltage line and a better transmission rate than with a 0.4 kV low voltage line.

More research work considering the 1000 V distribution system is needed. Some important areas in research and product development in the future are:

• development of 1/0.4 kV distribution substations o 1/0.4 kV distribution transformers o protection components o automation

• development of 20/1 kV distribution substations o protection components o automation

• final economical efficiency of the 1000 V system in different kinds of distribution networks

o for example in city areas • development of the whole distribution network when a 1 kV system is used

o use of underground cables o automation o small satellite model 110/20 kV substations

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BIBLIOGRAPHY

(Adato 2002) Adato Energia Ltd. Electricity and District Heating. Year book 2002. Downloadable: http://www.energia.fi/

(EMV 2002) Energiamarkkinavirasto. Sähköverkkoliiketoiminnan ns. tekniset tunnusluvut vuodelta 2002 –tilasto. Helsinki 2002. Downloadable: http://www.energiamarkkinavirasto.fi/

(EMV 1/2003) EMV 1/2003. Järventausta, P., Partanen, P., Mäkinen, A., Lassila, J., Nikander, A., Viljainen, S., Kivikko, K., Honkapuro, S., Sähkönlaatu jakeluverkkotoiminnan arvioinnissa. Lappeenranta University of Technology, Tampere University of Technology 2003.

(KA2:2003) KA 2:2003. Verkostosuositus Sähköenergialiitto ry. Verkostotöiden kustannusluettelo. Adato Energia Ltd. Helsinki 2004.

(Lakervi 1998) Lakervi, E., Holmes, E.J. Electricity Distribution Network Design, 2nd edition. Peter Peregrinus Ltd. 1995. Reprinted 1998. Short Run Press Ltd., Exeter England. ISBN 0 86341 309 9.

(LVD) LVD Low voltage directive 73/23/EEC. 19.02.1973. Downloadable: http://www.tukes.fi/sahko_ja_hissit/saadokset/73-23-ety_LVD.htm

(Sener 2002) Interruption statistics 2002. Sener ry. Finergy ry. series1. Adato Energia Ltd. Helsinki 2003. Downloadable: http://www.energia.fi/

(Trafotek 2003) Trafotek Ltd. Kujanpää, J. Phone discussion 28.10.2003. Trafotek Ltd. Kujanpää, J. Email 5.11.2003. Trafotek Ltd. Kujanpää, J. Email 26.11.2003. Trafotek Ltd. Lehtonen, J. Email 27.11.2003.

Overview Kaipia 1000 v Distribution

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