Power Electrical & High Voltage Engineering

a _ p l a c e _ t o _ s h a r e _k n o w l e d g e . . .

Distribution planning


1.0.0    Grid Substation

1.1.0    Kiribathkumbura Grid Substation

1.1.1    capacitor voltage transformer (CVT)

1.1.3    Bus bar

1.1.4    Isolators

1.1.5    Surge arresters

1.1.6    Auxiliary transformer

2.0.0    MV System

2.1.0    Single line diagram

2.2.0    Geographical maps

2.3.0    Gantry

2.4.0    Primary sub stations

2.5.0    Auto Re closer

2.6.0    LBS

2.7.0    ABS

2.8.0    DDLO

3.0.0    MV Distribution planning

3.1.0    Problems in existing system

3.2.0    Solution approach

3.3.0    Data collection

3.4.0    Modeling of network

3.5.0    Load allocation and load flow

3.6.0    Evaluation of energy demand

3.7.0    UTL – Utilization time of losses

3.8.0    Evaluation of loss of energy

3.9.0    Load forecast

3.10.0  Techniques in load forecasting

3.10.1  Time trend forecasting

3.10.2  Econometric forecast

3.10.3  Micro area load forecast

3.10.4  Requirement analysis

3.11.0  Solutions

3.11.1  Short term

3.11.2  Medium term

3.11.3  Long term

4.0.0    LV Distribution

4.1.0    ILV –  Improved Low Voltage

5.0.0    Provincial control centre

5.1.0    Reliability indices

5.1.1    SAIDI

5.1.2    SAIFI

5.1.3    CAIDI

5.2.0    Data Collection

6.0.0    Loss Reduction

6.1.0    Technical losses

6.2.0    Non-Technical losses

6.3.0    Methods used for reduce losses


Grid Substation

The main task of a grid sub station is to drop down high voltage in to medium voltage.

And here in central province all around its 132kV to 33kV conversion is done by Grid sub stations.

In central province there are seven grid substations. namely,

  1. Thulhitiya

3 x 132kV Incoming lines

3 x 31.5MVA Transformers

10 x 33kV Out going feeders

  1. Ukuwela

2 x 132kV Incoming lines

2 x 31.5MVA Transformers

8 x 33kV Out going feeders

  1. Rantembe

6 x 33Kv outgoing  feeders

  1. Kiribathkumbura

6 x 132kV Incoming lines

3 x 31.5MVA Transformers

10 x 33kV Out going feeders

  1. Mallawapitiya
  1. Wimala Surendra
  1. Habarana

1.1.0 Kiribathkumbura Grid Substation

This grid substation consist of three 31.5MVA This grid substation consist of three 31.5MVA, Δ-Y transformers. And the switching yard is consist with six incoming 132kV lines from

Ukuwela 2

Kothmale 1

Kurunegala 2

Kothmale 2

Ukuwela 1

Kurunegala 1

And there are 8 LT feeders such that,

H-2      New

H-3      New

H-4      Hanthana

H-5      Pallekele

H-6      Nawalapitiya

H-7      Sarasavi uyana

H-11    Kegalle

H-12    Spare

H-13    Kiribathkumbura

H-14    Back bone line to Mawanella

In kiribathkumbura gird substation, following instruments were used in switching yard,

1.1.1 capacitor voltage transformer (CVT)

CVT is a transformer used in power systems to step-down high voltage signals and provide low voltage signals either for measurement or to operate a protective relay. In its most basic form the device consists of three parts: two capacitors across which the voltage signal is split, an inductive element used to tune the device to the supply frequency and a transformer used to isolate and further step-down the voltage for the instrumentation or protective relay.

The device has at least four terminals, a high-voltage terminal for connection to the high voltage signal, a ground terminal and at least one set of secondary terminals for connection to the instrumentation or protective relay. CVTs are typically single-phase devices used for measuring voltages in excess of one hundred kilovolts where the use of voltage transformers would be uneconomical.

1.1.2 SF6 Breakers

These type of breakers used to break the circuit when in high voltages applied. SF6 gas has been compressed in the breaker. SF6 is a inert gas, hence the arc that generates when cct breaking is reduced.

1.1.3 Bus bar

Two series bus conductors are located to make the switching easy. It is useful when we want to disconnect an instrument or a transformer from the supply without an interruption to supply line.

1.1.4 Isolators

Isolators are used to safety of the system. These can not be used to break the line when it is loaded. It operates once SF6 or OCB’s cut the line. And we can get a visual certificate of line disconnection by isolators.

1.1.5 Surge arresters

Surge arresters are used for the protection of the line when there surges occurred. This instrument will ground the surge in such dangerous situations and protect the system. The theory is quite deep here.

1.1.6 Auxiliary transformer

Auxiliary transformer is there for the station supply. In kiribathkumbura there are three separate auxiliary transformers.


MV System

Here it is concerned 33kV system as Medium voltage system. And from grid substations to primary sub stations or to distribution transformers, it is fed by MV lines.

2.1.0 Single line diagram

We use single line diagrams to draw electrical distribution maps in the area. All three phases are represented by single line, hence it is called single line diagram. These diagrams are drawn only to get a rough idea of the distribution plan. It’s not according to a scale or with exact positions, directions.

2.2.0 Geographical maps

We do use geographical maps to locate positions which we represent by single line diagrams. We can use geographical maps to locate Land marks, Actual line lengths and to determine how to achieve places by roads. Most often 1:50,000 or 1:

2.3.0 Gantry

The purpose of a gantry is to distribute current path in to two or few lines in MV system. Most often it is single line dividend to two or three paths. It can be used to enhance the reliability and quality of the supply.

2.4.0 Primary sub stations

Primary sub station is to used for drop down 33kV to 11kV . in central province only three primary sub stations.  Which are used to facilitate Underground electricity network.

  1. Getambe

2 x 10MVA Transformers

2 x 33kV Incoming feeders

6 x 11kV Out going feeders

  1. Polgolla

2 x 5MVA Transformers

1 x 33kV Incoming feeders

7 x 11kV Out going feeders

  1. Bogambara

2 x 10MVA  Transformers

2 x 33kV Incoming feeders

6 x 11kV Out going feeders

2.5.0 Auto Re closer

Auto recloser is to used in systems to enhance the reliability and enhance protection of the line. The operation of an auto re closer is as intelligent switch. If any deficiency in the line current/voltage identified, the auto re closer disconnects the line and make it as it was in few minutes or pre determined time period. And if the obstacle is yet not fixed the AR again disconnects the line. This happens until the line getting  it’s normal situation and stop it’s process in a pre determined time period if the obstacle is not fixed. It is highly useful when the line draws through hygienic areas.

2.6.0 LBS

LBS Stands for Load Break Switch. Which means this switch can be used to break the line while it’s carrying load. But how ever in practice CEB is not using LBS to cut lines with it’s load since the un stability of the line.

2.7.0 ABS

Air Break Switch is meaning of this. And this is also can be used to cut the line while it’s having the load. The arrangement is consist of metal rods which cut’s the line with having a arc on air medium. No gas or oil medium used to reduce the arc.

2.8.0 DDLO

Drop Down Lift Off switch.  It is used as a protective device and as a switch in the distribution line. Its mechanism is to having a fuse inside a porcelain structure and with a conductive rod which acts as the switch. When the line having over current or an overload situation the rod drops down from the plug and disconnects the line where the protection occur. And some operator should fix the problem in the line and then lift off the rod to it’s plug to it’s normal operation.

Conductor types and sizes

Several conductor types are available to use in distribution.

Cross section area mm2 175 226.2 92.4
Overall diameter 18.3 19.53 12.3
Co efficient of thermal expansion  ˚C x 10-6 __ 19.1 19.1
Modulus of Elasticity kg/mm2 80,000 79,000
UTS kg/kgf . (N) 59105 79397 27204
Weight kg/km 551 842 319
Resistance 20’C – Ω/km 0.159 0.153 0.353
Maxm Current Rating A (@ 65˚C) 553 562 307


MV  Distribution planning

The continuous attention on growing power demand is highly essential in Electricity distribution industry. We have to do planning for next few decades in order to provide a quality, sustainable and un interrupted power supply to the consumer. And moreover , in MV system , the power loss is trying to keep less than 2%.

The distribution planning in Sri Lanka having following main objectives

  • To identify and formulate proposals for network reinforcement of the MV distribution system in order to provide electricity in sufficient quality and quantity for the development of the region
  • Voltage Improvement, Reliability Improvement, Catering for Adequate Network Capacity, Improving MV System Power factor.
  • To design the MV Distribution network to operate at optimal level of losses to ensure efficient system of electricity distribution

3.1.0 Problems in existing system

  • Poor voltage regulation
  • Low reliability
  • High system loss
  • In adequate capacity in MV and 33kV in feeds

3.2.0 Solution approach

  • Evaluate present situation
  • Forecasting for the future
  • Finding out resources and credibility and evaluate optimum solution
  • Finding out short, medium, long term solutions
  • Implementation

In CEB Distribution plan is to be revised once in every two years as a ten year rolling plan.

After finding the exact solution , we have to implement it as soon as possible to get the maximum profit out of it.

3.3.0 Data collection

  • Geological maps
  • Network data (conductor type, sizes, configurations)
  • Load data (distribution t/f, bulk supply, PSS)
  • Operational data (feeding arrangements, parallel/single operations)
  • Network development proposals (new GSS/PSS, new lines, Augmentations)
  • Development projects of the area (commercial, projects,)

The present situation of the area should be evaluated and the forecasting is done according to this present evaluation and collected data.

As the first stage of this planning process, the maximum (peak) power requirement from each grid sub for the province should be forecasted.

3.4.0 Modeling of network

A software package DIG is used to digitize the network to generate vector based map.  As scanning the machine can take the digitized image on to synerGEE and there after the data such as conductor size , type can be fed.

3.5.0 Load allocation and load flow

GSS and PSS feeder demands were taken at a Reference date during peak hours and it is used in the load flow analysis. But appropriate load growing factor is using when it is necessary. When committed loads are known, those loads with suitable adjustment factors were modeled in relevant years before load flow study was carried out.

3.6.0 Evaluation of energy demand

Using these formulae’s we can evaluate energy demands in the system.

3.7.0 UTL – Utilization time of losses

A hypothetical time derived such that losses during this time with a continues load equal to the peak load is as same as the losses of the system with actual loading over the day.

UTL  =  e2 ( 2 + e2 ) x 8760

( 1 + 2e)

3.8.0 Evaluation of loss of energy

The peak power loss is derived from,

E loss = P loss-pk x UTL per year

3.9.0 Load forecast

Load forecasting is the primary factor for future developments in the line. Therefore it is important to having it in most reasonable, accurate and realistic growth rates .

But however load forecasting is so uncertain because the presence of many uncertain factors.

There are two ways of doing this,

  • Top down
  • Bottom up

But since our forecasting is to be done in provincial vise , it is more reliable to use bottom up method.


3.10.0 Techniques in load forecasting

  • Time trend forecast
  • Econometric forecast
  • Micro area load forecast
  • Requirement analysis

3.10.1 Time trend forecasting

Historical load development is extend to predict the future demands

t   = no. of yrs

D  =  demand in tth year in GWh/yr

D1 = initial demand

g   = growth rate = em – 1

m  = gradient of the linear curve of log D  Vs  (t-1)

3.10.2  Econometric forecast

The electricity demand is actually depends on several key factors.

  • GDP growth
  • Population
  • Electricity price

And here a mathematical model has been developed to relate these factors with demand. And using past data a, b, c, d can be found.

Trend based growth rates

Province Domestic/commercial sector (%) Industrial sector (%)
2004-08 2009-13 2004-08 2009-13
West (north) 8 5 7 5
Central 6 5 4 3
Eastern 6 5 5 4

Forecast of peak power demand for an improved network (MV)

Province\year 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013
West(north) 275 293 314 336 361 379 390 427 432 453
Central 128 134 141 149 158 165 172 178 186 194
Eastern 78 79 84 89 94 98 103 109 114 119
Total 481 506 539 574 613 642 665 714 732 766

3.10.3 Micro area load forecast

This method is mostly used for urban areas where there is a pre drawn plan available for development. In this method different zones are identified for different types of economic activities such as residential, commercial, industrial, etc. different load densities depending on the economic activities are applied to different zones to arrive the final demand figure. This method has been used in developing a load forecast of the Colombo city.

3.10.4 Requirement analysis

The electricity requirement for a given area is assessed to determine the future electricity requirements. This assessment includes the household electricity requirements, assessment of future industrial loads, etc.

3.11.0 Solutions

In solution space, there are three categories.

3.11.1 Short term

Implemented within about in one year time with CEB’s own resources.


  • Completion of on going distribution network reinforcements
  • 11kV to 33kV conversion
  • MV like augmentation(short lines)
  • Interconnection lines
  • New 11kV/33kV in feeds
  • Unmanned PSS
  • Capacitor placement
  • Voltage regulators

3.11.2 Medium term

Need 3-5 years to implement.


  • Back bone lines and gantries
  • MV line Augmentation (long lines)
  • Enhancement of primary feeding facilities/voltage regulators
  • Requirement of 33kVnew in feeds (Grid sub’s )
  • Enhancement of grid substation facilities

3.11.3 Long term –

Need about 5-10 years to implement.


  • New grid substations
  • New feeder bays


LV Distribution

By low voltage we refer to 230V distribution system. This is directly connected with domestic consumers.

We have to collect data from the field. Line voltages, line currents, consumer density etc. and then draw the sketch of the area and feed to the computer. We can use synerGEE to analyzing the system.

Present loss of the LV system is about 8.9%

If no improvement done in 5 years time LV system loss will be 11.8% With improvements , in 5 years time , LV system loss will be about 5%

This shows the significance in doing improvement to the system.

Ploss = I2R ;   consumer      ;  I         ; Ploss

When the no. of consumers are increased , the power loss also increasing.

4.1.0 ILV – Improved Low Voltage

Normally if the phases are not well balanced or the load is high, the voltage at the terminals would be lower. Therefore low voltage improvement is needed.

For this , first we are getting data from the field by a field visit. Customer density , line currents and voltages will be valuable in this regard. Then we have to draw the area map and fed in to SynerGEE software. By SynerGEE we can analyze the network about low voltages and system losses , etc. so then we can find the optimal solutions to minimize the loss and maximize the customer satisfaction.


Provincial control centre

The provincial control centre for central province is situated in getambe PSS. All the line interruptions in the central province should be reported to provincial control centre. And if any one in CEB wants to take a line for a interruption for a maintaining or any other purposes , he should take permission from provincial control centre.

Provincial control centre should maintain a good communication between all the depot’s in the province and provincial office. And should also maintain good record of provincial breakdowns, failures,  and all the details that may neccasary to plan for a good supply.

5.1.0 Reliability indices

The system reliability is measured from previous values of system interruptions. It is necessary to take measurements in system reliability for improving system qualities. And moreover we can compare the situation of our system with others with aid of reliability indices.

5.1.1 SAIDI –  system average interruption duration index.

UNITS = Hrs/consumer/month

5.1.2 SAIFI –   system average interruption frequency index.

UNITS = Interruptions/consumer/month

5.1.3 CAIDI –  customer average interruption duration index.

Sri lanka’s  Central Provincial SAIDI SAIFI Figures for year 2008

January 4.7 0.1 12.46
February 2.8 0.7 13.78
March 6.8 4.7 12.26
April 8.3 1 17.76
May 3.0 1.1 13.24
June 5.6 2.1 18.13
July 4.3 1.2 20.81
August 3.6 1.4 15.13
September 3.2 0.8 17.4
October 4.3 0.7 15.2
November 5.5 N/A N/A
December 4.1 N/A 11.1

5.2.0 Data Collection

Provincial control center keeps following data for future evaluations and estimates to be done.

  • Feeder tripping information
  • MV interruption arranging
  • MV break down
  • System loses


Loss Reduction

Losses place a significant stage in present day situation. Because , if we can reduce the losses , we can reduce the selling price and especially  we could save power and save money.

Losses are in two phases ,

  • Technical losses
  • Non technical losses

6.1.0 Technical losses

  • Conductor resistance losses
  • Bi metalic effect losses
  • Un balanced loss – neutral current loss
  • Joint/ Jumper losses
  • Hygenic problems
  • Earth  faults

6.2.0 Non-Technical losses

  • Instrumental losses/Meetering errors
  • Meter readers errors
  • billing errors
  • Theft

6.3.0 Methods used for reduce losses.

  • Reconductoring can be done to reduce the conductor resistance loss.
  • We should pay attention on system phase current balancing. If we reduce the neutral current , we would be able to maintain the system un-balance loss to be minimum. This should be considered when new connections are made.
  • When joints and jumpers are done, always there appears additional resistance. The resistance can be minimize by using modern technological crimping clamps for joints. Therefore it is better to use H-Connectors for joints always to reduce improper joint losses.
  • The transmission lines should be well cleared from trees and possible earth faults. Otherwise the earth fault loss will be high and also line tripping might be occur.
  • Continuous attention on meter values and billing pattern can be reduce possible theft.
  • Adequate instrument, meter testing and proper training to meter readers might reduce metering losses

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8 Responses to “Distribution planning”

  1. […] Distribution planning […]

  2. mithra said

    Its a fruitfull site.

  3. Nice document. I studied more about power system in sri lanka

  4. nice document n very useful

  5. Dhanushka jayasekara said

    very useful

  6. anyaogu henry said

    Good material, well written.

  7. Sriyanganie Ariyarathna said

    Very useful document to improve knoweledge of MV distribution system in Sri Lanka.

  8. Shehan Vimukthi said

    The document is extremely useful.. Helped me a lot to understand some facts easily.. Would have been much handy if it included some photographs (just a suggestion) …

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