EA Capacity Selection

Capacity Calculation

For capacity calculation following procedure is applied:

  1. Assessment of Load: 

    1. Linear Load: EI, lift, pump, AC

    2. Non-linear Load: Power panel, battery charger, UPS, exchange.

      Problems associated with non-linear load: 

      1. Hunting.
      2. Voltage fluctuation.
      3. Harmonics.
      4. Cross-Magnetization / De-Magnetization

      Remedy: 

      • Engine Side: 

        1. Use better grade of governing.
        2. FIP (Fuel injection pump) setting at 110 %.
      • Alternator side: 

        1. Use of damper winding.
        2. Using PMG (permanent magnet) type alternator.
        3. Using next higher frame size. (Normally alternators are designed to take 40 % non-linear load).

      Methods to reduce non-linearity: 

      1. Reducing unbalanced thyristor pulses.
      2. Using balanced thyristor load.
      3. Using proper filter.
      4. Restricting non-linear load to 40 %.
      5. Using AVR-GCU to improve stability.
      6. Bypassing the AVR when EA set is in use.
  2. Selection of Alternator: Normally alternators are designed to take about 25 % overload for short time, but owing to the limitation of the prime mover, it gets limited to the 10 %. For use in the Department, 10% overload capacity for 1 hour in twelve hours running is a must. After arriving at the load, suitable derating factor is applied to get the rating of the alternator.

    The rating of an AC alternator is normally expressed in kVA and kW. The output of an Alternator (Electrical Machines) are specified at 1000m altitude, 40 °C ambient and + 5 % voltage variation. For conditions other than this derating factor has to be applied. This is due to the fact that AC Generators when running on load generates heat. The difference between winding temperature and local ambient temperature is termed as temperature rise. The temperature rise of the motor/generator winding is given by the following formulae:

    t2 = { R2/R1 × (t1 + 235) – 235}, where t1 & t2 are the temperature before and after stopping in °C and R1 & R2 are the winding resistance in Ω at temperature t1 & t2.

    For the healthy operation of the Generator, this temperature rise has to be limited to permissible limits. This limit depends on the type of insulation material used and the surrounding air temperature where the Generator is working. Permissible temperature rise for various insulation classes with the ambient of 40° C and altitude of 1000m are given in the table below:

    Insulation ClassAEBFH
    Permissible Temperature Rise60 °C75 °C80 °C105 °C125 °C

    For other altitude levels the permissible temperature rise may be taken from the following diagram:

    Normally B/F classes of insulation are used for the winding of the stator and rotor.

    The derating factor on account of temperature is given in the table below:

    Ambient Temperature40 °C45 °C50 °C55 °C60 °C
    Derating1.00.960.920.880.84

    Effect of Altitude: The air takes away the heat generated inside the Generator. As the altitude increases the density of air decreases which results in a higher temperature inside the Generator. Up to an altitude of 1000 m above sea level, the change in the density of air is insignificant and does not change its heat transfer properties. Hence all Industrial Generators are designed for operations suitable up to 1000 m altitude. For higher altitudes, the generator has to be derated and the derating figures are given in the table below:

    ALTERNATOR DERATING ON ACCOUNT OF ALTITUDE
    Altitude in m above sea levelDerating Factor
    10001.000
    11000.994
    12000.989
    13000.984
    14000.979
    15000.974
    16000.969
    18000.959
    20000.949
    22000.938
    24000.927
    26000.916
    28000.906
    30000.890
    32000.880
    34000.870
    36000.860
    38000.840
    40000.826

    Effect of Humidity: With the use of the proper grade insulating material and insulating varnish, the Generators are fully ‘tropicalised’. However in places where condensation occurs, it is necessary to fit ‘anti-condensation heaters’ in the Generators. The purpose of this heater is to keep the winding of the generator warm when it is not running and therefore to prevent the lowering of the insulation resistance below the safe value.

  3. Selection of Engine: To select the capacity of the prime mover, the Efficiency of Alternator (η) is considered to be 0.90. Subsequently derating factors are applied to get the capacity of the Engine.

    DERATION OF ENGINES: For derating of Engines following formulae is used:

    Derating Factor (K) = [{Px – a.ϕx.Psx} / { Pr – a.ϕr.Psr}]m x [Tra / Tx]n × [Tcr / Tcx]q

    Where r stands for reference condition i.e. if rating is for NTP then Pr = 760 mm of Hg, Tr (Tra)= 300 K (27 °C), Humidity = 50 %, but normally 60 % is tolerated by all engines.

    x stands for site condition, so Px and Tx are to be noted.

    Psx = Saturation vapour pressure at site condition.

    Psr = Saturation vapour pressure at ref. Condition.

    Tcx = Std reference absolute charge air cooled temperature.

    Tcr = Charged air cooled temp at site condition

    For naturally aspired engine : a & q = 0, m,n = 1

    For turbo charged engine w/o air cooling : a = 0, q = 1, m = 0.7, n = 2

    For turbo charged engine with air cooling : a = 0, q = 1, m = 0.7, n = 1.2

    At higher altitude derating is due to the lack of oxygen. It effects most to naturally aspirated engines and least effected are Turbo Charged Engines and they do not have any effect upto 500m. Due to higher temperature, output of Engine is reduced.

    Also the tables regarding deration of diesel engine outputs are given below:

    DERATION OF DIESEL ENGINE OUTPUTS
      2 / 4 stroke engine, naturally aspired or supercharged without after cooler4 stroke TC without after cooler 4 stroke TC with after cooler 
    Altitude: For every 500 m above 160 m from mean sea level3.5 %2.5 %2.5 % 
    Inlet air temperature: For every 10 °F above 85°F2.0 %3.0 %Nil
    Temp of cooling water to charge air cooler: For every 10°F above 85 °FNilNil3.0 %
    DERATION OF DIESEL ENGINE OUTPUT FOR HUMIDITY
    Atmospheric temperature (°F)Percent Humidity
    102030405060708090100
    85     0.51.01.52.02.4
    90    0.41.01.62.22.73.3
    95   0.20.91.62.22.93.64.2
    100   0.71.52.23.03.84.65.3
    105  0.31.22.13.03.94.85.76.6
    110  0.71.82.83.84.95.96.98.0
    115  1.22.43.64.86.07.28.49.6
    120 0.41.73.14.55.97.38.610.011.4
    125 0.82.33.95.57.18.710.211.813.4
  4. Miscellaneous: After the selection of engine and alternator, other components like diesel tank, exhaust pipe etc are selected.

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