New 20kV Network Concept for an Existing Distribution Network

7/27/2019 New 20kV Network Concept for an Existing Distribution Network

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2010 China International Conference on Electricity Distribution 1

CICED2010 Session x Paper No xxx Page1/7

New 20kV Network Concept for an Existing Distribution Network

Dr. Holger Mller1, Theodor Connor

11, Network Consulting, Power Technologies International (PTI) Siemens AG, Erlangen, Germany

E-MAIL: [email protected], [email protected]

Abstract: This paper presents a study of a distribution

network of the large city in a former republic of the Soviet

Union. The current situation is analyzed and a new network

concept is developed.

The major part of the current electrical network has been

built in the 1960th and 70th using old Russian technology. The

economy develops strongly in the last years, and it can be

estimated that the total consumption will roughly double

within the next 15 years.

Today the distribution network is using several voltage levels.

A lot of the electrical equipment in the network is reaching the

end of its lifetime in the coming years. Therefore and due to

the historical developments in the network structure,

reliability is low compared to European standards.

For these reasons a new network concept has been developed

in a consulting project undertaken by Siemens PTI and

described in this paper.

Especially for the new and optimal network concept for the

city, the reduction of the large number of existing voltage

levels has been proposed by introducing a new 20 kV level.

The Greenfield planning approach is used for this task to

achieve the optimal solution for the network.

Finally a way is presented to transform the current network

step-by-step into the recommended 20 kV solution with future

oriented and optimized network structure for the year 2025.

The objective of these steps is to achieve a fast improvement of

network reliability.

Keywords: Greenfield planning, distribution network,optimal network concept.

1. Introduction

The liberalization in many countries all over the world has

led to competition between companies of the electricity industry,

which has created challenges as well as opportunities for both

utilities and industrial consumers.

After years of small investment in the electrical infrastructure

many customers face now challenges for future operation of their

networks:

increasing consumption aging equipment movements of load centers target to use standardized equipment flexible adaptation to changes in load and structureFurthermore old equipment and grown network structures

make the maintenance and operation of the network difficult and

expensive. Especially fault detection and protection coordination

within these networks are very complicated. Large-area outages

are often the results of this historical development causing

discontent with public consumers and high interruption costs for

industry consumers.For strategic network planning the countrys economy is an

important factor. Based on the positive economic development in

the past with growth rates of about 10%, the standard of living

shall be further developed. Thus it can be derived that the total

consumption will double within the next 15 years.

On the other hand if security of supply can not always be

guaranteed, poor security of supply can have a negative impact on

the economic development. Thus a highly reliable network is an

important basis for the future economic development.


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2 2010 China International Conference on Electricity Distribution


2. Methodology

2.1 Planning approach

The main objective of network planning is to achieve a

coordinated development of the electrical distribution network

towards a secure, reliable, efficient and economical performance.

Based on the data collected of the actual status of the network

and considering the future development of the city, the load

growth of the network can be estimated over the next years. The

current status of the actual system is analyzed and the major

problems are determined.

Using the Greenfield planning approach a network concept

for the complete distribution network is developed without

considering the existing structure and constraints. In this way the

optimal target network design for the long term development is

achieved. Based on this solution for the year 2025 and taking into

account the situation of the actual system status, future

modifications and a sequence of steps can be derived to transform

the actual system in to the target structure. Thus a consistent

network design for the distribution network can be achieved

A typical planning loop is shown in Fig. 1.

Datacollection

Datacollection

ActualSystem

ActualSystem

Longterm

Longterm

Shortterm

Shortterm

Mediumterm

Mediumterm

Datacollection

Datacollection

ActualSystem

ActualSystem

Longterm

Longterm

Shortterm

Shortterm

Mediumterm

Mediumterm

2025

network

Next

steps

Weak

points

Figure 1. Planning Loop for Greenfield Planning Approach

2.2 Planning criteria

The planning criteria describe the standards for development

and design of distribution networks. The compliance of the

proposed concept with these criteria is investigated through

calculations of the network performance.

The main planning criteria are: Reliability criteria: the network should comply with the

(n-1)-criteria for disturbances as well as for planned

outages and main reliability indices should show a good

performance.

Thermal limits: thermal capacity of equipment must notbe exceeded under normal operation and contingency

situation at planning process.

Voltage ranges: a voltage range of 95% to 105% ofnominal voltage should be maintained at normal

operation; at contingency situation a voltage range of

90% to 110% of nominal voltage is acceptable.

Short-circuit current levels: maximum short-circuitcurrents have to be lower than rating of network

components.

Economical network structure: network planning has tolead to a cost-effective power system considering both

economic investments and efficient network operation.

Losses and other operational costs have to be minimized

efficiently.

3. Status of Medium Voltage Network

At the beginning of the strategic network planning process

starts with an extensive data collection. To get an overview of the

current status of the future development, the network under

investigation is the Medium Voltage (MV) distribution system of a

large city with nearly 1 million inhabitants and an area of about

100km.

3.1 Development of Load

By analyzing the peak load of the city of the last years the

growth of the load for the future years can be estimated. To

achieve a more accurate evaluation, information about new

industrial and housing areas as well as data about the economic

growth (e.g. the gross national product GNP) is important.

If no detailed data of the load distribution over the city is

available, the actual load distribution and its development can be

approximated using topographic maps and satellite photos. This

can help to subdivided and distinguish areas of different load


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density. Here the following categories are used: Residential areas Business and governmental areas Industrial areasUsing this approach the complete city could be divided into

several sectors with a certain area and a designated load density.

The results matched the collected data very well.

Figure 2. Example of Dividing the City into Sectors with typical

Usage and Load Density

3.2 Analysis of Existing Assets

The condition and age of the existing assets in the network,

like switchgear, cables and transformers, are assessed. The

analysis of the 110 kV and 35/10 kV components gives a very

good overview of the state of the network. The location of old

equipment indicates areas, where there is a strong need for action.

For this network the results showed, that a quarter of the

110/35 kV substations are over 40 years and 50% are between 20

and 40 years. Thus a lot of network components will have to be

replaced in the coming years.

3.3 Analysis of Present Network Performance

An initial analysis of a systems existing structure from the

HV transmission grid to the MV distribution levels shows the

actual network performance and reveals any existing weaknesses

or bottlenecks. There are some factors, which have a negative

impact on the network performance.

The main issue is the age of the network components leading

to a higher number of faults and disturbances in the network.

The current network concept is characterized by longoverhead lines inside the city and T-off connection with only a

very low number of circuit-breakers, with a very simple protection

concept and without any automated resupply of loads after faults.

These characteristics cause high number of interruptions and long

down times for all customers.

Additionally the currently large load growth results in

network equipment, which is already loaded to their maximum

capacity. Thus in some areas new customers can not be connected

anymore.

Another factor is the high number of voltage levels from

220 kV and 110 kV via 35 kV and 10 kV to 6 kV and 400 V. A

reduction of voltage levels and an economic optimized selection of

suitable voltage levels is important to reduce maintenance costs

and providing higher flexibility for network operation.

4. Optimal Selection of Voltage Levels

To develop different possible alternatives, which can be

analyzed and compared, and to determine the optimal network

concept, a Greenfield planning approach is used. The

performance is always judged using the planning criteria described

above. After the final concept is determined, it is investigated

using detailed network models and state of the art analysis tools.

Here the network simulation tool PSSSINCAL has been used.

4.1 Selection of Optimal Medium Voltage Level

To ensure an economical operation of the system, the

reduction of the existing voltage levels of 220 kV, 110 kV, 35 kV,

10 kV and 6 kV to a maximum number of three is necessary. Here

three different alternatives are compared.Alternative 1 220/35kV:

The existing 35 kV network level will be used to substitute

the existing smaller 10 kV and 6 kV levels. This voltage level is

well suitable to supply large loads, e.g. large hotel or business

complexes with around 3-5 MW of peak load. These loads can

reasonably not be supplied anymore by the existing 10/6 kV

voltage levels.

Although a parallel 110 kV and 35 kV voltage level is not

economically feasible, as the voltages are very similar. Thus the

Industry

Housing

Business


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220 kV level will have to substitute the extensive 110 kV voltagelevel within the city.

This solution has some major drawbacks. One is the 35 kV

level itself, as this is not a standard voltage level used throughout

the world. Thus the equipment is expensive. Also there is only a

small number of manufacturers for 35/0.4 kV substations.

Additionally the complete 110 kV network has to be replaced by

220 kV substations and lines, which will also have a high impact

on the total costs.

Alternative 2 220/110/10kV:

Another possibility is to upgrade the old 35kV level by

110kV components. This can happen in a step by step approach

and the existing 10kV network can be used and extended.

A drawback is that the 10kV level is very limited, when in

new city developments large loads have to be connected. Here

only a few loads in the MW range can be supplied via on

110/10kV substation. Also the number of cables needed in parallel

is quite high.

This means that the 10kV level is already today meeting its

limitations. Thus it is not an adequate solution for future

developments in the city, but it can be used as intermediate step to

a network with higher voltages.

Alternative 3 220/110/20kV:

As a third possibility it is investigated to extend the existing

110kV network to replace the old 35kV level. A new 20kV voltage

level will then substitute the 10kV and 6kV levels.

Today 20kV is a standard level for distribution network in

large areas of the world. A large number of manufacturers is

existing, thus the components are having moderate prices. The

power capability is high compared to the 10kV solution, suitable

to supply all different kinds of loads within a city. Hence the

voltage level is adequate also for future developments in larger

cities.

Compared to alternative 2 this solution shows initial higher

investment costs, as a new voltage level has to be introduced. On

the other hand the advantages for the network are larger then in the

other options. As a lot of components are reaching the end of their

lifetime in the next years, it is a good opportunity to set the course

for a suitable distribution network.

4.2 Development of Suitable Network Concept

The future network concept has to be suitable to provide

energy to the customer in a highly reliable way with low losses

and without too complex network structures.

Therefore several alternatives have been analysed to find an

optimal concept, keeping in mind the current situation and already

existing developments.

The existing 220kV network and the 220/110kV substation,

forming a ring around the city, are used as the basis of the final

concept. From these HV substations at the citys border the power

is transmitted to the city centers via 110kV cables or overhead

lines. These lines are supplying the 110/20kV substations in a ring

structure.

Advantages of this concept (shown in Fig. 3) are a rather

simple structure, where the substations are fed from one main

220/110kV station.

110 kV

220 kV

110 kV

20 kV

Main Infeed

Substation

RMU20 kV

0.4 kV

Legend:Circuit-breaker (n.c.)Circuit-breaker (n.o.)

Figure 3. Schematic Overview of the Developed Concept

Thus the short-circuit currents are well within the ratings of

typical 20kV switchgear. Throughout the network structure typical

component ratings are used, e.g. the standard transformer size for a

110/20 kV transformer is selected to 31.5 MVA.


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If a fault occurs on one line in the 110kV network, thedistance protection or cable differential protection will trip the line

without an interruption of loads. Thus the concept shows a very

good reliability performance. In case of critical loads the structure

can be enhanced to ensure also (n-2) security, e.g. for the airport or

for governmental buildings.

4.3 Optimal Location of Substations

If new substations will be build to serve new loads or to

replace old substations, the optimal location of the station has to

be determined. In an analysis the loads within one or two sectors

are combined in the centers of gravity for the supply areas. The

load centre governs the optimal site of the new substation with its

planned standardized equipment.

An example graphic from the software PSSSINCAL

showing the load density of several areas within the city is shown

in Fig. 4, where green indicates a low value and red indicates a

high value for the load density. The middle of the red area

symbolizes the optimum location for the substation in that area.

Figure 4. Load Density and Optimal MV-Substation Location

The optimum indicates the location, which will lead to a

minimum length of MV cables required to supply the loads and

subsequently a minimum of losses during operation. These

locations have of course to be matched with local constraints and

restrictions.

5. Verification of Network Performance

To analyze the behavior of the system of the developed

network concept, the complete network from the surrounding

220kV level down to each 20/0.4kV substation is modeled in the

power system simulation software PSSSINCAL. Here the

geographic information can also be included to obtain a network

model with correct position of the components. With this model

detailed calculations have been performed to ensure a good

performance of the proposed network concept.

Load-flow analysis assures that the voltages at bus bars in all

voltage levels and loadings of all components are within the given

limits during normal operation as well as during disturbed

conditions. Additionally the losses are evaluated and the tap range

of the transformers is approved.

To confirm the ratings of switchgear and cables,

short-circuit calculations are performed to assess the maximum

stress on the components during faults. Additionally maximum

and minimum short-circuit currents are used to determine the

necessary protection concept and settings.

Extensive probabilistic reliability calculations are performed

to ensure a high security of supply and good reliability indices, e.g.

low frequency of interruption and customer minutes lost.

6. Roadmap and Future Steps

To improve the situation in the areas determined in the

assessment, actions have been work out and proposed for the areas

with the highest rating. Therefore the following aspects have been

described, which are important for the identified areas:

Location of new substation depending on required space Load growth inside area up to year 2025 including new

development areas

Disentanglement of areas fed by one substation at theMV and LV level

Fast improvement of reliabilityIn the following one example of fast improvement of power

supply within one area of the city is described.

In Fig. 5 the old network structure with its old substations

(red circles) is shown. Today these substations S/S 1..3 are

connected via T-off connected lines causing unreliable energy


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supply for these substations due to the lack of circuit-breakers andprotection devices.

Figure 5. Old network structure (red) with three substations and

T-off at the 110kV level

To increase the reliability a new substation S/S 1 can be

placed on the optimal location (blue circles in Fig. 6). The new

substation will have state of the art circuit-breakers and protection

devices with remotely controlled breakers.

A step by step replacement and extension of the network will

finally lead to the proposed network concept in the coming years.

Figure 6. New network substations

7. Conclusions

This paper describes the development of a new network concept

based on a Greenfield planning approach. In the beginning the

current state of the network has been analyzed and major

drawbacks have been identified. Also the planned development of

the loads has been investigated and the load growth up to the year

2025 has been estimated.Based on predefined planning criteria different possible concepts

and voltage levels combinations have been developed and

compared. The optimal solution with the substitution of the

existing 6 kV and 10 kV voltage levels by the newly introduced

20 kV level has been discussed and proposed.

A detailed analysis of the network in the power system simulation

software PSSSINCAL ensured the good performance of the

proposed concept. Also the main targets to improve reliability and

security of supply and to reduce of operational costs are achieved.

Based on the developed concept a roadmap is described for

transforming the current network into the new structure. First steps

are described for a selected number of areas with a high priority

for improvements.

The proposed concept for the year 2025 is the base for enabling

the distribution network to fulfill its tasks in the future, coping

with the future load growth, ensuring efficient operation and

guaranteeing a high quality of living.

Biography

Dr. Holger Mueller, born in 1973 in Darmstadt, received

Dipl.-Ing. and Dr.-Ing degrees from the Technical University of

Darmstadt in 1999 and 2003 respectively. His PhD research

covered modeling and dynamic simulations of HVDC systems.

Since 2008 he works with the network consulting department of

Siemens PTI in Erlangen, Germany, as a senior consultant in the

field of power system analysis, mainly in the fields of transmission

and distribution planning and smart grid solutions. His research

activities also include aspects about large scale integration of

renewable energies into power systems.

Email: [email protected].

Theodor Connor was born in 1953. He received his

Dipl.-Ing. degree in Electrical Engineering from the Technical

University of Berlin in 1980. Since 1980 Mr. Connor is working

for Siemens AG in Erlangen. As Head of Network Consulting his

areas of expertise are strategic planning of transmission,

distribution networks, grounding and interference. Mr. Connor is

doing training courses worldwide. He is a member in different

technical committees, e.g. DKE, CIGRE,CENELEC and IEC.

Email: [email protected]

S/S 2

S/S 1

S/S 3

S/S 2

S/S 1

S/S 3


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New 20kV Network Concept for an Existing Distribution Network

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