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Alternative paths & reserve capacity

The main task of every electric power system is to provide its customers with a reliable and economically feasible supply of electricity. In order to provide a reliable supply of electricity, there is a built in reserve capacity within the grid to cope with contingencies, increased demand and scheduled maintenance.

Reliability of the electric power distribution system for 6 alternative loop configurations
Reliability of the electric power distribution system for 6 alternative loop configurations

This reserve capacity must be designed and constructed in the most economically and technically optimal way in order to ensure reliable power transmission as well as the lowest possible cost.

Reliability is defined by the International Electrotechnical Commission (IEC) as: “the ability to perform a required function under given conditions for a given time interval“. Reliability of the electric power system can be increased by either shortening the duration of the interruptions of the power supply or by lowering the frequency by which interruptions occur.

The probability that a component in the system will fail is generally increased when the number of components rises.

By introducing alternative paths and reserve capacity, the disruption time for the customers and SAIDI (System Average Interruption Duration Index) will be reduced. This reserve capacity may be utilised under a couple of hours every year which leads to that the reliability of the system may be decreased.

By calculating the impact of using reserve capacity and its impact on SAIDI, it is easier to determine if the risk taken is acceptable. Risk is defined as the “effect of uncertainty on objectives” which implies that risk can both have positive and negative implications.

Sectionalised loop whose first section is used as a test system for Markov models
Figure 1 – Sectionalised loop whose first section is used as a test system for Markov models

Reserve alternatives to loop configurations

This thesis is focused on analysing reserve alternatives to loop configurations and their impact on SAIDI in the electrical distribution system (EDS). The results are used to propose dimension criteria that can be used by the DSO in the general case of the medium voltage distribution system in Gothenburg.

The medium voltage distribution system in Gothenburg is composed mostly of 11 kV underground cables and it is the different configurations of the underground cables that has been analysed.

The EDS in Gothenburg is supplied by a number of 130/11 kV distribution substations and supplies over 1600 secondary 11/0.4 kV substations. Since the customer and load concentration differs throughout the Gothenburg area, three areas will be analysed and examined. The geographical areas that will be examined are City, Urban and Industry.

Provided statistics from GENAB are used as input in the models to provide a high correlation between the models and reality. One part in maximising the reliability and utilising the reserve in an efficient manner is to use probabilistic methods to analyse the probabilities of potential faults and contingencies.

By performing simulations and creating simplified models of the system it is possible to acquire the data needed to make motivated and analysed decisions on utilisation and future investment of the reserve capacity.

The dimension criteria include the maximum amount of 11/0.4 kV secondary substations per loop that can use the same reserve as well as including the eventual implications on SAIDI and reliability associated with the utilisation. These criteria are presented for different cases that are applicable in the Gothenburg region.

The criteria should also take current carrying capacity of the underground cables and the resulting voltage drop into consideration. By using appropriate models of the power system to portray the different cases, the impact on SAIDI (System Average Interruption Duration Index) could be calculated.

Title:Reliability of the Electric Power Distribution System for Alternative Reserve Configurations; Master’s Thesis in Electric Power Engineering – Aron Von Scheele at Department of Energy and Environment, Electric Power Engineering, Chalmers University of Technology
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Reliability of the electric power distribution system for 6 alternative loop configurations
Reliability of the electric power distribution system for 6 alternative loop configurations

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  1. Bisi Ogi-Olu
    Aug 13, 2019

    Thanks a lot EEP Editorial Team for the article.I wish to request the comment or, Umesh Maharaja to educate us on other indices like CAiDi, CAiFi, SAidi to highlight how loop automation is important for system reliabilty
    My regards; Bisi Ogi-Olu Ibadan , Nigeria

  2. Mark McCloy
    Aug 13, 2019

    Who is responsible for monitoring, measuring, recording and reporting these guidelines? I must assume it will be reported to the local Utility.

  3. Umesh Maharaja
    Aug 12, 2019

    SAIDI is indicator of reliability of supply , it does not reflect the spare capacity of the system . Pl give reference of Other IEEE indices like CAiFi, CAiDI and SAidi ,to highlight how loop automation is important for system reliability improvement. Thanks

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