Building Services Engineering Design 2 Module BNEE483

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					         Building Services Engineering Design 2
                   Module BNEE483

   Hugo Gallagher

   Room M709


   Tel: 0141 331 8836

   Lecture 2
            Maximum Demand
   This is the maximum load on a system
    during any specified period (half-hour

   Since electrical energy can not be stored
    on a significant scale it is also, by
    definition the maximum output of the
    system during that period.
                  Load Factor, L
   Defined as:

                Average demand
                Maximum demand

   It is always less than 1
   A typical value for load factor, L is 0.35
                Load Factor
   Load factor, depending on the
    requirements of consumers, is not under
    the direct control of the supply authority
    but it has a considerable effect on the
    ultimate cost per unit of electricity.

   Total capital expended on plant, as stated
    previously forms a standing charge.
       Average load factor in the UK?

   The average load factor of generation
    plants in the UK was 66.3% in 2004.

   This means that, on average, the output
    from power stations during 2004 was
    66.3% of the total maximum possible
    from every plant.
                      Power Factor
   In ac circuits:   Power (watts) = volts x amps x power factor

                              i.e. P = VI cos ϕ

   With constant voltage V, it will be seen that a low power factor will
    require a high current to flow for a given power, but current will be
    reduced if power factor is improved, i.e. brought nearer to unity.

   Thus a low power factor causes excessive current to be drawn for a
    given power and this in turn will require plant and equipment of a
    higher current rating. It is for this reason that plant and equipment
    are rated in kVA and not kW.

   To enable plant rating to be kept within rating and also have spare
    capacity, the supply authority encourages consumers to improve
    their power factor by having the maximum demand charge per kVA
    instead of kW.
              Power Factor
   Thus for a given kW power demand, a low
    power factor will give a high kVA
    maximum demand charge.

   This is the usual method of including
    power factor into the tariff, but sometimes
    the tariff will include a penalty clause for
    poor power factor working and sometimes
    a Varmeter will be installed and a charge
    made for reactive kilovolt amperes.
           Why improve Power Factor ?
   Improving a systems power factor will reduce the total power
    consumed by an electrical installation and will provide the following

   Financial saving - By reducing power consumed electricity costs are

   Extended equipment life - Reduced electrical burden on cables and
    electrical components

   Increase load capacity - Provide additional capacity for other loads
    to be connected

   Environmental benefit - Reduced power consumption means less
    “Greenhouse” gas emissions and fossil fuel depletion by power
            Diversity Factor (D)
   As stated earlier, the total charge is based on a standing charge and
    a running cost charge, i.e. a two part tariff..

   The standing is determined by plant size and that in turn, is
    determined by the load connected by the consumers after applying
    a diversity factor.

   This factor takes into account the fact, that it is most unlikely that
    all connected plant will be in use at any one time.

D      = sum of consumer demands
        Actual system maximum demand

   The figure is always greater than 1 (typically 2.5)
                   Interest Cost
   The cost of building the scheme is obtained by

   It is usually possible to borrow the money at a fixed rate
    of interest. The cost per annum in interest is thus:

   Capital cost (amount borrowed) x rate of interest (%)

   Thus if 1M was borrowed at 10% p.a. the total cost in
    interest p.a. would be £1,000,000 x 0.1 = £100,000
             Depreciation Cost (1)
   Since equipment will wear out and eventually
    have to be replaced, a “sinking fund” must be
    established to pay for repairs and replacement.

   In order to arrive at a p.a. value for this, the
    “lifetime” of the equipment must be estimated.

   The cost p.a. in depreciation is thus:

                 Capital cost
              Assumed “lifetime”
                Depreciation Cost (2)
   Thus equipment costing £1 M with an estimated lifetime of 20 years
    would require a “sinking fund” of p.a.

                          £1,000,000 = £50,000 p.a.
 In this way, a sum of £1 M would be built up to enable either
(a) the equipment to be replaced, or
(b) the amount borrowed to be repayed, at the end of the equipment’s

Hence the total transformer costs per annum are:

   Interest costs + depreciation costs

   In the above example these amount to £150,000 p.a. or a total %
    p.a. of 15% for interest and depreciation.
   In UK, electricity is generated/produced at power generating stations at
    25 kilovolt (KV) potential, in a 3 phase supply at 50 cycles per second

   Thereafter it is processed by step-up transformers to 132, 275 or 400 kV

   before connecting to the “National Grid”.

   Vast and complex network of overhead lines and underground cables
    carrying power at high voltages to centres of high load density.

   Energy at high voltages transmitted by the National Grid system is then
    fed into grid substations for the transformation of the transmission
    voltage to secondary transmission voltages of 132 KV and 33 KV.
   Secondary transmission is for large consumers
    such as factories and those in areas of high load

   Further substations reduce the secondary
    transmission voltages to 11kV.

   The term 'distribution' is usually used to refer to
    the feeding of electrical energy through
    overhead lines and underground cables to
    supply small industrial, commercial and domestic
              Supply Authority
   Comply with the requirements of the Electricity
    Supply Regulations.

   This states that the supply authority must
    constantly maintain the type of current (dc or
    ac), the frequency and the declared voltage.

   Frequency, however, may be varied by 1%; the
    declared voltage can be allowed to vary by 6%.
       The Electricity Supply Acts
   The Electricity Supply Acts defines the voltages of

   Extra-low voltage                        30V (ac) or 50dc

   Low Voltage                              250V or less

   Medium Voltage                  Between 251V and 650V

   High Voltage                     Between 651 and 3000V

   Extra-high Voltage                      Exceeding 3000V
The figure below summarises the pattern of electricity
  generation, transmission and distribution today   .
         Introduction to Power
   An electrical network initiates at the point of

   Electrical power is generated by converting the
    potential energy available in certain materials
    into electrical energy.

   This is either done by direct conversion of kinetic
    energy, e.g. wind or water turbines, or creating
    steam to drive the turbines, e.g. coal or nuclear
A typical
    Generation of Electrical Powers
   The electrical powers generated are either transferred onto a bus to
    be distributed (small scale), or into a power grid for transmission
    purposes (larger scale).

   This is done either directly or through power transformers,
    depending on the generated voltage and the required voltage of the
    bus or power grid.

   Power transmission, whereby the generated electrical potential
    energy is transmitted via transmission lines, usually over long
    distances, to HV substations.

   HV substations will usually tap directly into the power grid, with two
    or more incoming supplies to improve reliability of supply to that
    substation’s distribution network.
             Electrical Transmission
   Electrical transmission is normally done via high to extra high voltages, in
    the range of 132 – 800 kV.

   Mega volt systems are now being developed and implemented in the USA.

   The longer the distance, the more economical higher voltages become.

   Normally, the transmission voltage will be transformed at the HV substation
    to a lower voltage for distribution purposes.

   This is due to the fact that distribution is normally done over shorter
    distances via underground cables.

   The insulation properties of three-phase cables limit the voltage that can be
    utilized, and lower voltages, in the medium-voltage range, are more
    economical for shorter distances.
Schematical illustration of a
    typical Power Grid.
         Critical Medium Voltage
   Critical medium-voltage (MV) distribution
    substations will generally also have two or more
    incoming supplies from different HV substations.

   Main distribution substations usually supply
    power to a clearly defined distribution network,
    for example, a specific plant or factory, or for
    town/city purposes.
             Power Distribution
   Power distribution is normally done on the
    medium-voltage level, in the range of 6.6
    – 33 kV.
   3-ph power is transferred, mostly via
    overhead lines or 3-core MV power cables
    buried in trenches.
   Single-core-insulated cables are also used,
    although less often.
   LV distribution is also done over short
    distances in some localized areas
      Power Distribution Network
   A power distribution network will therefore typically
    include the following:

   HV/MV power transformer (s) (secondary side)
   MV substation and switchgear
   MV power cables (including terminations)
   MV/LV power transformer (s) (primary side)

   The distribution voltage is then transformed to low
    voltage (LV), either for lighting and small power
    applications, or for electrical motors, which is usually fed
    from a dedicated motor control center (MCC).
Typical Power Distribution
                     Voltage Levels
   Voltage levels are defined internationally, as follows:

   Low voltage: up to 1000 V
   Medium voltage: above 1000 V up to 36 kV
   High voltage: above 36 kV

   Supply standards variation between continents by two general
    standards have emerged as the dominant ones:
   In Europe
   IEC governs supply standards
   The frequency is 50 Hz and LV voltage is 230/400 V

   In North America
   IEEE/ANSI governs supply standards
   Frequency is 60 Hz and the LV voltage is 110/190 V.
                    Overhead Lines
   Overhead lines are far cheaper than underground cables for long
    distances, mainly due to the fact that air is used as the insulation
    medium between phase conductors, and that no excavation work is

   Support masts of overhead lines are quite a significant portion of
    the costs, that is the reason why aluminum lines are often used
    instead of copper, as aluminum lines weigh less than copper, and
    are less expensive.

   Copper has a higher current conducting capacity than aluminum per
    square mm, so once again the most economical line design will
    depend on many factors.

   Overhead lines are by nature prone to lightning strikes, causing a
    temporary surge on the line, usually causing flashover between
    phases or phase to ground.
                 Line Insulators
   Line insulators are normally designed to relay
    the surge to ground, causing the least disruption
    and/or damage.

   This is of short duration, and as soon as it is
    cleared, normal operation may be resumed.

   Sophisticated auto-reclosers are employed on an
    increasing number of overhead lines.
    Overhead Lines - Advantages

   Less expensive for longer distances

   Easy to locate fault.
Overhead Lines - Disadvantages

   More expensive for shorter distances
   Susceptible to lightning
   Not environment-friendly
   Maintenance intensive
   High level of expertise and specialized
    equipment needed for installation.
Underground Cable Installations
   Underground (buried) cable installations are
    mostly used for power distribution in industrial

   They have the following properties:

 Less expensive for shorter distances

 Not susceptible to lightning

 Environment-friendly
 Not maintenance intensive.
Underground Cable Installations


   Expensive for long distances
   Can be difficult to locate fault.

   A substation can be defined as any
    premises or part of premises in which
    electrical power is transformed or
    converted to or from high voltage or
    which contains high voltage switchgear.

   Particular types of substation can be
    identified as follows:
      (1) Distribution substation (DSS)

   A substation which has a function of
    distribution only, with or without voltage

   Any substation which controls an incoming
    Supply of electrical energy from another
    system or from the electricity company is
    excluded from this category.
        (2) Intake substation (ISS)

   A substation, with or without voltage
    transformation, which has the function of
    controlling the incoming supply of
    electrical energy from another system or
    electricity company.
        (3) Standby substation (SBSS)

   A substation, with or without voltage
    transformation, which has the function of
    controlling the supply of electrical energy
    from standby generators
         Siting an Intake Substation
   For large buildings or small sites where the ISS is the only substation its ideal location
    is at the load centre.

   In practice, difficult to achieve as account must be taken of: Physical and structural
   Need for access for conveying and unloading (and replace at a later date) of heavy
    equipment, e.g. switchgear and transformers
   Fire and explosion risks, especially where oil immersed equipment is to be used.
   In most large buildings, ISS will generally be located at ground floor, although with
    the introduction of non-oil equipment high level substations are becoming more

   Small sites a compromise for the ISS position has to be sought between the load
    centre, access problems and the point of supply from the electricity company.
   In large sites where the ISS performs its controlling function in association with one
    or more DSS's its siting is less critical.

   In the majority cases it will be near the site boundary and positioned to take account
    of access and point of supply from the electricity company.
               Siting a Standby substation
   More complex than that of an ISS
   depends mainly on whether there is to be a single large central standby or multiple
    small individual standby supplies.
   Where the decision is to utilise multiple small LV generating sets then these will be
    sited at individual load and will not truly be standby substations.
   With large central standby facilities the ideal situation, considering its control and
    electrical protection, is to combine the ISS and SBSS.

   However this may not be possible due to the following:

   Noise problems associated with generators:
   Desire to keep the two sources of supply (REC and STANDBY) electrically and
    physically remote to ensure that the loss of any noise, however catastrophic and
    cause, does not affect the other.
   In determining the ideal position the 'factors to be considered are as follows:

   As for ISS
   The need to store and handle fuel for the prime mover
   Noise nuisance.
     Siting a distribution substation
   Ideal position for a Dist. Substation is at the load
    centre of the area it is to serve.

   Seldom possible to achieve and in practice a site
    should be chosen that gives the best compromise
    between the following:

   Siting of the various loads
   Access road for heavy equipment
   Fire and explosion risks
The substation forms a node point in the electric
Substation equipment :
  Transformer to change the voltage and current level.

   Circuit breaker (CB) to interrupt the load and fault current. The fault
    current automatically triggers the CB.

   Disconnect switch to provide visible circuit separation. Permit CB
    maintenance. No load operation.

   Voltage and current transformers to reduce the current to 5 A,
    and the voltage to 120 V, and to insulate the measuring circuit from
    the high voltage

   Surge arresters for protection against lightning and switching
    overvoltages. They are voltage dependent, non linear resistance.
Arial View of a Substation
            Substation Circuit Diagram
 Supply     S           Transmission

                                                       Bus 1

                           CBA 4                     CBA 1               switch

Circuit breaker
                           CBA 5                     CBA 2
                                                             Voltage     Circuit
                                                           transformer   breaker

                           CBA 6                     CBA 3               Disconnect
                                                                                       Circuit breaker
                                                             Bus 2                     assembly (CBA)

                                               Grounding disconnect

                                          Surge arrester
       T3                            T2
                Transmission lines
Transformer   Surge


              Circuit Breakers
   A CB is a switching device built ruggedly
    to enable it to interrupt/make not only the
    relatively large load current, but also the
    much larger fault current which may occur
    on a circuit.
   These are designed for asymmetrical
    faults, which are more severe than the
    symmetrical faults due to dc off-set of the
    fault current.
Circuit Breaker Concept
                          Fixed contact

                 Switch Closed

 Moving    Arc            Fixed contact

                                          SF6 injection

                 Switch Opens
SF6 Circuit Breaker
    SF6 Circuit Breakers
   The superior arc-quenching ability of SF6
    gas can be attributed to the fact that it is
    electronegative, which means that its
    molecules rapidly absorb the free electrons
    in the arc path between the breaker
    contacts to form negatively charged ions
    which are ineffective as current carriers.
     Advantages of SF6 Circuit Breakers
   Reliable current interruption, no restriking
   Quiet operation
   Closed gas circuit keeps interior dry, so that
    there is no moisture problems
   Little erosion because of short arc time
   No carbon deposit
   As the CB is totally enclosed and sealed
    from atmosphere, suitable for use in coal
    mines; explosive hazard areas.
       69 kV substation

                                    Bus bar

             Current CT
Disconnect                Circuit         Disconnect
500 kV Circuit breaker
Disconnect Switch

Surge Arrester
Shunt reactor protected by Lightning
            Transmission Lines
Distribution line (4.2-45 kV)

   Wood tower with cross arm. The     Typical distribution line
    wood is treated against rotting.
   Simple concrete block foundation
    or no foundation.
   Small porcelain or plastic post
   The insulators shaft is grounded
    on important lines to eliminate
    leakage current causing wood
    tower burning.
   Simple rod grounding.
   Shield conductor is seldom used.
Radial Distribution system
                                                 Sub-transmission Line

                                                                         Circuit Breaker

     Feeder 1                                                                     Feeder 4

                                      Feeder 3
             Feeder 2

                                                                                  Radial Feeder

        Radial Feeder          Fuse


   To Consumer
   Service Drop
                                  Three-phase Four-wire
                                      Main Feeder
 Line                                Cable and
                                  transmission line

Fuse cutout


                     12.47 kV
                                Service Drop

  Sub-transmission and Distribution line

    Fuse and disconnector
                                    Distribution line 13.8 kV

Distribution Cable 13.8 kV
   Telephone line

                             240/120V line