The Electrical Distribution Architecture – Part Seven

In the previous Topic “ The Electrical Distribution Architecture – Part One “, I listed the Tasks required for application of Electrical Distribution architecture design process, they were: 
  • Assigning of electrical installation characteristics, 
  • Assigning of Technological characteristics, 
  • Using Architecture assessment criteria, 
  • Step (1): Choice of distribution architecture fundamentals, 
  • Step (2): choice of architecture details, 
  • Step (3): choice of equipment, 
  • Recommendations for architecture optimization. 

And I explained the first two tasks: Assigning of electrical installation characteristics & Assigning of Technological characteristics in the following previous topics:

  • The Electrical Distribution Architecture – Part Two
  • The Electrical Distribution Architecture – Part Three
  • The Electrical Distribution Architecture – Part Four
  • The Electrical Distribution Architecture – Part Five
  • The Electrical Distribution Architecture – Part Six



  • Today, I will explain The Using Architecture assessment criteria as follows.



    Third: Using Architecture assessment criteria


    The Architecture assessment criteria is Certain decisive criteria which are assessed at the end of steps 1, 2 & 3 listed above , in order to validate the architecture choice.


    Effect of Architecture assessment criteria on the Electrical Distribution architecture design process:
    According to the result of discussions between the customer, architect and other parties based on Architecture assessment criteria and after applying the Optimization recommendations, it may be possible to loop back into step (1), step (2) or step (3) and revalidate the architecture choice.



    The Architecture assessment criteria include the following categories or levels of priority: 


    1. On-site work time, 
    2. Environmental impact, 
    3. Preventive maintenance level, 
    4. Availability of electrical power supply. 


    1- On-site work time


    There is a relationship between a project’s time to completion and its cost; this relationship can be one of the following:

    1. Direct proportional relation, 
    2. Direct trade-off. 

    And by understanding the customer requirements for the time-cost relationship, the designer can be able to implement these requirements on the Electrical Distribution architecture design process.

    The following table summarizes the customer requirements for the time-cost relationship as levels of priority:



    #
    Characteristic
    Definition
    Level Of Priority
    1
    On-site work time
    Time for implementing the electrical equipment on the site.

    Secondary:
    the on-site work time can be extended, if this gives a reduction in overall installation costs,

    Special:
    the on-site work time must be minimized, without generating any significant excess cost,

    Critical:
    the on-site work time must be reduced as far as possible, imperatively, even if this generates a higher total installation cost,

    And according to the selected level of priority, which is based on the customer requirements for the time-cost relationship, the configurations of the Electrical Distribution architecture, MV/LV equipments and feeding system can be changed.



    2- Environmental impact


    The purpose of Environmental impact assessment is to ensure that decision makers consider the ensuing environmental impacts when deciding whether to proceed with a selected project design or it needs re-design.

    The following table summarizes the different levels of priority for Environmental impact assessment:



    #
    Characteristic
    Definition
    Level Of Priority
    2
    Environmental impact
    It is an assessment of the possible positive or negative impact that a proposed project may have on the environment, social and economic aspects.

    Non significant: environmental constraints are not given any special consideration,

    Minimal:
    the installation is designed with minimum regulatory requirements,

    Proactive:
    the installation is designed with a specific concern for protecting the environment. Excess cost is allowed in this situation. E.g.: using low-loss transformers


    The environmental impact of an installation will be determined according to the method carrying out an installation lifecycle analysis, in which we distinguish between the following (3) phases:
    1. manufacture, 
    2. operation, 
    3. end of life (dismantling, recycling). 

    In terms of environmental impact, 3 indicators (at least) can be taken into account and influenced by the design of an electrical installation. Although each lifecycle phase contributes to the three indicators, each of these indicators is mainly related to one phase in particular: 

    1. Consumption of natural resources mainly has an impact on the manufacturing phase, 
    2. Consumption of energy has an impact on the operation phase, 
    3. “recycleability” potential has an impact on the end of life. 

    The following table details the contributing factors to the 3 environmental indicators:




    Indicators
    Contributors
    Natural resources consumption
    Mass and type of materials used
    Power consumption
    Joule losses at full load and no load
    «Recyclability» potential
    Mass and type of material used



    3- Preventive maintenance level

    The reliability of Electrical Distribution architecture only lasts as long as the shortest component/equipment life in the system.

    The designer philosophy must address this issue by reducing the number of component/equipment that needs to be replaced, thus decreasing the chance of a failure and increasing system reliability.


    The following table summarizes the design philosophy levels for preventive maintenance characteristic:



    #
    Characteristic
    Definition
    Choice
    3
    Preventive maintenance level
    Number of hours and sophistication of maintenance carried out during operations in conformity with manufacturer recommendations to ensure dependable operation of the installation and the maintaining of performance levels (avoiding failure: tripping, down time, etc).

    Standard:
    according to manufacturer recommendations.

    Enhanced:
    according to manufacturer recommendations, with a severe environment,

    Specific:
    specific maintenance plan, meeting high requirements for continuity of service, and requiring a high level of maintenance staff competency.

      

    4- Availability of electrical power supply


    A lot of disturbances can affect any electrical network operations, these disturbances can be:
    • brown-outs, 
    • over-voltages,
    • voltage distortion, 
    • voltage fluctuation, 
    • voltage imbalance. 

    These disturbances presented above may affect: 
    • Safety of human life,
    • Safety of property,
    • The economic viability of a company or production process.

    therefore, Disturbances must be eliminated.

    Definition of Availability of electrical power supply: 

    This is the probability that an electrical installation be capable of supplying quality power in conformity with the specifications of the equipment it is supplying.

    So,

    Availability level (%) = (1 - MTTR/ MTBF) x 100 


    Where:


    MTTR (Mean Time To Repair): the average time to make the electrical system once again operational following a failure (this includes detection of the reason for failure, its repair and re-commissioning),

    MTBF (Mean Time Between Failure): measurement of the average time for which the electrical system is operational and therefore enables correct operation of the application.

    MTBF = cumulative operating hours/( number of outages + 1)



    Methods for increasing Availability of electrical power supply: 



    A number of methods exist to limit the risk of power outage as follows:

    • Division of the installation so as to use a number of different sources rather than just one,
    • Subdivision of the installation into priority and non-priority circuits, where the supply of power to priority circuits can be picked up if necessary by another available source,
    • Load shedding, as required, so that a reduced available power rating can be used to supply standby power,
    • Selection of a system earthing arrangement suited to service-continuity goals, e.g. IT system,
    • Discrimination of protection devices (selective tripping) to limit the consequences of a fault to a part of the installation.

    Note that the only way of ensuring availability of power with respect to utility outages is to provide, in addition to the above measures, an autonomous alternate source, at least for priority loads. (See Fig.1). 

    Fig (1): Priority and Non-Priority Circuits

    This source takes over from the utility in the event of a problem, but two factors must be taken into account: 

    • The transfer time (time required to take over from the utility) which must be acceptable to the load 
    • The operating time during which it can supply the load. 


    Availability of electrical power supply Categories: 


    The different availability categories can only be defined for a given type of installation. E.g.: hospitals, data centers.

    Example of classification used in data centers:
    • Tier 1: the power supply and air conditioning are provided by one single channel, without redundancy, which allows availability of 99.671%, 
    • Tier 2: the power supply and air conditioning are provided by one single channel, with redundancy, which allows availability of 99.741%, 
    • Tier 3: the power supply and air conditioning are provided by several channels, with one single redundant channel, which allows availability of 99.982%, 
    • Tier 4: the power supply and air conditioning are provided by several channels, with redundancy, which allows availability of 99.995%. 



    In the next topic, I will explain Step (1): Choice of distribution architecture fundamentals. So, please keep following.



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