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Distribution and Transmission Systems

- A Comparison

Anders Wikstrom

A-EES-9811

October 8, 1998

1 Introduction

The aim of this report is to point out differences and similarities between trans-mission and distribution systems that might be important to consider whenplanning, analyzing, optimizing or designing power systems.

In [2] the following definitions can be found:Distribution system (power operations)That portion of an electric system which delivers electric energy from transfor-mation points on the transmission or bulk power system to the customer.Transmission system (power operation)An interconnected group of electric transmission lines and associated equipmentfor the movement or transfer of electric energy in bulk between points of supply

and points for delivery.The original task for the transmission system was to transport bulk power

from, often remote, power stations to the load centers. As the power system hasdeveloped, new tasks have occurred. As power systems were connected, spinningreserves could be shared and excessive power in one system could be transferredto another. This led to a more economic operation of the power systems. The re-cent de-regulation of many energy markets has turned the transmission systemsinto a market place.

Also the distribution systems have seen some changes. As the use of embed-ded generation has increased, distribution systems nowdays can inject powerinto, and not just distribute power from, the transmission system. For this rea-son distribution system sometimes are referred to as local networks. Another

area for the use of distribution networks is communication, both for remotecontroll of loads and computer communication.The report starts with comparing physical factors such as voltage levels and

structure. Therafter dynamics, reliability and power quality are considered. Inthe last section some economical comparisons are made.

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2 Physichal factors

2.1 Voltage levels

Different maximum voltages are used in transmission systems in different coun-tries. 800 kV is used in Brazil, 765 kV in USA and 755 kV in Canada, but500 kV is the most common maximum voltage.

In Sweden, the voltage levels of the transmission system are 400 and 220 kV.The local distribution systems have voltages of 20 kV down to 0.4 kV. In betweenthe transmission and distribution system there are subtransmission systems (re-gional networks). These have voltage levels at 130, 70 and 40 kV. [3]

2.2 Power levels

As the transmission system transfer the bulk supply to the customers, the powerlevel in a single transmission power line is much higher then in a cable in thedistribution system. In [1] it is said Depending on the number and size ofconductors per phase and the distance involved, a single 400 kV transmissioncircuit could carry the output of a 2000 MW power station. In a cable in adistribution system at 0.4 kV carrying a current of 50 A the transmitted powerwould be around 35 kW.

2.3 Structure

In [1] different topologies for distribution networks are discussed. These areshown in Figure 1, where substations are drawn as circles.

According to [4], the most common topology in European distribution sys-

tems is radially operated ring networks (see Fig 1 d, called open loop in [1]),while transmission systems are meshed.

In [1] some criteria for which topology to choose are mentioned, e.g. wantedreliability, possibilities for later extensions, topography of the countryside, costof equipment and earlier used designs.

In [4] the criteria are divided into external and internal criteria. As externalcriteria standard voltage levels and public restrictions are mentioned. Internalcriteria discussed are e.g. reliability of different parts of the network, power andsupply quality.

According to [6], more then 90 % of the Swedish transmission and distribu-tion system at 20 kV and above is built with overhead lines (OHL). Distributionsystems in urban areas are mostly cables and the the percentage of OHL will

continue to decrease. The reason for this is visual and public worries fromelectromagnetic fields. [5]

3 Generation

The power generated in power plants in remote areas are transferred to loadcentres by the transmission system. It is getting more common with embeddedgeneration, i.e. smaller power plants connected to the distribution systems.

Ref. [5] shows that in five European countries examined, the capcity ofembedded generation increased from 3000 MW 1983 to 8000 MW 1993. Themain type of generators are hydro generators, wind turbines and gas engines.

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e

Open point

a b

c d

Figure 1: a: Meshed network b: Interconnected network c: Link arrangementd: Open loop e: Radial system

The following problems for network planning are pointed out for the increasingshare of distributed generation:

The network dimensioning could be wrong if it is based on recorded loaddata that include a significant share of embedded generation. The dimen-sioning maximum load would then be higher than the recorded value.

The supply and power quality could decrease due to starting and stoppinggenerators. Furhermore, windturbines could cause problems with risingvoltages and flicker.

The safety of maintenance staff could be affected, if the protection ofthe generators does not trip the generator during faults. Then the staffrisks entering a live working site that is fed backwards from embedded

generators.

4 Reliability

In [1] the following is pointed out as the main factors to judge the reliability tocustomers:

frequency of interruptions

durations of each interruption

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Voltage level [kV] Lines & Cables [MSEK] [%] Stations [MSEK] %

400-220 20 000 24 7 000 22130-70 10 000 11 7 000 2240-10 20 000 24 7 000 22

0.4 35 000 41 11 000 34

Total [MSEK] 85 000 32 000

Table 1: Value of investments at different voltage levels. [3]

Lines & Cables [MSEK] Stations [MSEK]Voltage level [kV] Operation Maintenance Operation Maintenance

400-220 50 (5 %) 20 (8 %) 120 (20 %) 60 (27 %)130-70 40 (4 %) 10 (4 %) 230 (37,5 %) 40 (19 %)

40-10 440 (45,5 %) 130 (48 %) 140 (22,5 %) 60 (27 %)0.4 440 (45,5 %) 130 (48 %) 120 (20 %) 60 (27 %)

Total [MSEK] 970 290 610 220

Table 2: Costs for lines & cables and stations at different voltages. [3]

the value a customer places on the supply of electricity at the time thatthe service is not provided

A failure in a transmission system will affect more customers then one in adistribution system. Transmission systems are therefore operated as meshednetworks with more than one possible way to transfer the energy.

The use of ring networks in distribution system makes it possible to isolatefaults and, through reconnections in the network, minimize the impact of thefault for the customers.

5 Dynamics

In power systems care must be taken to avoid dynamic phenomena that canlead to instability and colapse of the system.

In distribution systems static equations are often sufficient.

6 Power quality

The definition for power quality used in [7] is: Any power problem manifestedin voltage, current or frequency deviations that results in failure or misoperationof customer equipment. Problems that occur are e.g. harmonics, voltage dips,voltage fluctuations or transients.

6.1 Interaction between loads and network

Sources of quality problems are often loads. Harmonics are caused by converters,flouroscent lightning and arc furnaces. Non-linearities in transformer magneti-zation and rotating machines do not cause significant harmonics while operating

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Voltage level [kV] Losses [GWh]

400-220 2700 (25 %)130-70 2300 (21,5 %)40-10 3400 (32 %)

0.4 2300 (21,5 %)

Total 10 700

Table 3: Losses for different voltage levels. [3]

under normal steady state conditions, but during transient disturbances theycan increase their harmonic contribution considerably. [8]

In distribution systems converters can be found e.g. in machine drives andcomputers. Transmission systems can have HVDC stations that causes harmon-

ics. [9]Harmonics might not be a problem in the transmission system itself, but

harmonics can be fed in from a distribution system and transfered to anotherpart of the power system. [10]

Voltage dips are short duration reduction in the rms voltage. They aremainly caused by short circuits and starting of large motors. Adjustable-speeddrives, process-control equipment and computers are very sensitive to dips, andthe equipment may trip when the rms voltage drops below 90 % for longer thenone or two cycles. [16]

Flicker is observable fluctuating light intensity caused by fluctuations inthe voltage amplitude. Sources of voltage fluctuations can be arc furnaces orwind turbines. [11]

Frequency deviations are caused by unbalance between produced and con-sumed power.Transients can be caused by switching operations, e.g. capacitor switching.

6.2 Requirements

The recommendations given in Swedish Standard [14] show some of the require-ments for distribution systems. The harmonic content, measured as mean valuefor three seconds, are 4 % for odd harmonics and 1 % for even harmonics. Theselimits are valid up to the seventh harmonic. For higher harmonics it is just saidthat they should be lower than the given limits. The highest allowed total rela-tive harmonic content is 6 %. For single, non repetitive cases, e.g. transformerconnections or start of engine, the relative harmonic content may shortly be

higher then the given limits.The voltage level should always be in the interval 207-244 V at the customer.The highest allowed frequency deviation is 0,5 Hz.

7 Economy

The figures in this section are taken from [3] and they give the value in SEKin 1990 so the relative investments (in %) might be more interesting then theactual values.

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Voltage level [kV] Stations Lines Total

220-400 0,2 0,5 0,7130-70 0,2 0,2 0,4

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taxes

In Stockholm the prices in September 1998 was 0.336, 0.277 and 0.190 SEK/kWhfor the base tariff. [17]

Svenska Kraftnat (SvK) has a tariff for use of the Swedish transmissionsystem. The tariff has four parts:

power fee

special power fee

energy fee

investment contribution

The power fee is the main part of the tariff (60 %). It is based on the power levelthe customer uses. As the main power flow is from north to south, it is moreexpensive to inject power in the north than in the southern parts of Sweden,and consumption is more expensive in south than north.

The special power fee is new (included since 1998-01-01). It is supposedto cover the costs SvK has for the reserves. It is just a few percent of the totalprice.

The energy fee is based on injected or consumed energy. It is calculated asthe product of the coefficient of losses, the price of energy and injected/consumedenergy. The coefficient of losses varies geographically between 10 %. Negativecoefficient means that the customer get paid from SvK, as an injection in thesouthern parts of Sweden decreases the power flow from north to south andtherefore also decreases the losses.

When a new connection requires investment in new equipment, SvK cancharge a contribution fee to cover their costs. [13]

7.3 Regulatory constraints for companies

In Sweden the electricity law regulates the trading of electric energy betweencompanies and customers. The law states the network owner is allowed to chargea reasonable fee for the use of their network. Whether the fee is reasonable ornot can be tested in a court. The Swedish National Energy Administration(Statens Energimyndighet) is the authority that supervises the network compa-nies tariffs. They assume that the distribution companies can lower their tariffswith 20 %. [12]

Net owners can not refuse to connect small scale (less then 1500 kW) powerstations to their network unless there are certain reasons.To be able to participate on the deregulated market and be able to change

supplier a customer must have an energy meter that reports the consumedenergy on an hourly basis. The maximum price for the energy meter is setto 2500 SEK by the government. There are discussions about changing thisdecision and use the Norwegian model insted, i.e the customers are billed aftertheir expected consumption.

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8 Concluding remarks

As described here there are some differences between distribution systems andtransmission systems. These might be important to take under considerationfor example when applying techniques developed for a transmission system ona distribution system or vice versa.

References

[1] E. Lakervi, E. J. Holmes, Electricity distribution network design, 2nd Edi-tion. Exeter: Peter Peregrinus Ltd 1996. ISBN 0 86341 309 9

[2] IEEE Standard Dictionary of Electrical and Electronics Terms, 4th Edition.New York: Institute of Electrical and Electronic Engineers, Inc. 1988. ISBN

1-55937-000-9

[3] Vattenfall Elkraft 91 Sektorsutredningen om elkraftteknisk forskning ochutveckling i Sverige 1991. (in Swedish)

[4] Unipede Distribution Study Committee Group of Experts Network Con- figuration and Design Network Design - Applied Practices in European

countries

[5] Unipede Distribution Study Committee Group of Experts Network Con- figuration and Design Distribution Network Configuration and Design,

Likely trends in distribution systems

[6] Svenska ElkraftforeningenPower Distribution in Sweden

Stockholm: Stel-lan Stal tryck 1987. ISBN 91-7622-055-9

[7] R C Dugan, M F McGranaghan, H W Beaty Electrical Power SystemsQuality

[8] J Arrillaga, D A Bradley, P S Bodger Power System Harmonics

[9] F Jonas Measurement and Calculation of Harmonic Impedances of a 400kV Network close to an HVDC Station

[10] Discussion with Erik Thunberg

[11] T Larsson Voltage Source Converters for Mitigation of Flicker Caused byArc Furnaces. PhD thesis Stockholm: KTH 1998. ISBN 91-7170-274-1

[12] Natmyndigheten Nyhetsblad fran Natmyndigheten Nr 6 1997(www.stem.se) (in Swedish)

[13] Svenska Kraftnats Homepage (www.svk.se)

[14] Svensk Standard SS 421 18 11 Spanningsgodhet i lagspanningsnat forallman distribution (in Swedish)

[15] G Andersson, M Ghandhari, A Herbig, L Jones, D Lee Power Flow andStability Control in Power Systems Department of Electric Power Engi-neering, KTH

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[16] M H.J. Bollen Power Quality 14, 15 16 September 1998 (Course material)

Department of Electric Power Engineering, Chalmers University of Tech-nology

[17] Bill from Stockholm Energi

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