Introduction While   parked   at   the   airport,   all   passenger   aircrafts   need   to   be   provided   with   the   essential   utilities   such   as   electric   power   and   conditioned   air.   This   is accomplished by utilizing the aircraft on-board equipment, the airport portable power equipment or the airport ground power system. The   use   of   stationary   Ground   Power   System   significantly   improves   the   ground   service,   reduces   the   amount   of   airport   portable   equipment,   improves   the comfort   of   passengers,   improves   the   environmental   management   and   allows   for   compliance   with   the   requirements   of   current   standards.   The   implementation of   stationary   Ground   Power   System   has   positive   environmental   impact,   by   eliminating   the   use   of   portable   diesel-powered   equipment   and   the   on-board Aircraft Power System (APS). In the result a significant reduction of NOx and CO2.emission can be accomplished. Aircraft Electrical Systems The   following   is   a   brief   description   of   aircraft   electrical   systems.   The   purpose   of   this   introduction   is   to   present   design   concept   and   unique   challenges associated with the implementation of electrical apparatus and equipment in aircrafts. Modern aircrafts are equipped with the advanced power generating and distribution systems, which are used to power: -        Flight instruments -        Subsystems necessary for the safe operation of aircraft propulsion, navigation, and controls. -        Passenger services such as cabin lighting, environmental control, preparation of food, entertainment equipment, etc. As   aircrafts   fly   faster   and   grow   larger,   the   demand   for   power   and   the   electric   power   distribution   system   grow   more   complex.   In   passenger   aircraft   this   means more   power   to   the   galley   units,   environmental   control   and   entertainment   systems,   while   military   aircraft   require   more   power   for   sensors   and   weapon systems.   In   both   categories,   there   is   also   an   increased   power   demand   for   actuators,   lighting   systems,   avionics   and   heating.   Several   primary   and   redundant backup generator systems are used to power the aircraft. The   AC   generators   which   are   directly   connected   to   the   aircraft   jet   engines   usually   provide   the   primary   power.   Commercial   aircraft   and   many   military   planes also   use   the   Auxiliary   Power   Unit   (APU).   Essentially   APU   is   an   electrical   generator   powered   by   a   dedicated   small   jet   engine.   This   generator   is   always   in operation to supplement the primary power supply or replace it in a case of main engine failure. Many   types   of   aircrafts   also   carry   an   additional   Ram   Air   Turbine   (RAT)   that   can   be   deployed   when   needed   to   provide   an   emergency   power.   The   purpose   of a RAT is to keep critical systems operating long enough to land safely. Aircraft   electrical   subsystems   operate   on   many   different   voltages   both   AC   and   DC.   However,   most   of   aircraft   systems   use   115   V   AC   at   400   Hz   or   28.5   V DC.   A   26   V   AC   is   also   used   in   some   aircraft   for   lighting   purposes.   The   higher   frequencies   (400   Hz)   allows   for   use   of   smaller   and   lighter   power   supplies   and equipment.   The   400   Hz   electrical   systems   offer   a   distinct   advantage   over   60   Hz   used   in   utility   power   generation.   The   aircraft   space   is   at   a   premium   and   the weight   is   a   critical   element   effecting   engine   thrust   and   fuel   burn   and   in   effect   the   aircraft   range.   However,   this   higher   frequency   power   sources   generate higher voltage drop. There are two types of voltage drops: resistive and reactive. The   resistive   losses   components   are   a   function   of   current   flowing   through   a   conductor   and   vary   with   length   and   size   of   the   conductor.   The   higher   frequency is not a factor in respect to resistive voltage drop because of the short transmission range. The   reactive   voltage   drops,   on   the   other   hand,   are   caused   by   the   inductive   properties   of   the   conductor.   Reactive   drops   are   a   function   of   both   cable   length and   the   AC   frequency   flowing   through   the   conductor.   With   high   frequencies   such   as   400   Hz,   the   reactive   voltage   drop   is   up   to   seven   times   greater   than   at 60 Hz. Typically,   the   commercial   jet   aircraft   electrical   equipment   is   power   by   a   400   Hz   power   generating   system   with   300%   redundancy.   The   No-Break-Power- Transfer   is   used   to   eliminate   power   interruption   during   the   load   transfer.   The   power   requirement   of   aircraft   distribution   system   depends   on   the   size, manufacture,   and   model.   Typically   is   in   the   range   from   30   KVA   to   180   KVA.   The   latest   versions   of   Airbus   A380   and   Boeing   787   are   equipped   with   much larger   power   plants.   This   is   a   result   of   the   application   of   "fly-by-wire"   technology,   wider   use   of   electric   actuators   and   the   increase   in   the   passenger   cabin power demand. Gate Ground Power System for Servicing Aircrafts In   modern   large   airports,   the   electric   power   for   handling   and   operation   of   aircrafts   at   gates   is   provided   from   the   external   ground   source   often   refer   to   as Ground   Power   System   or   Ground   Power   Supply.   A   ground   power   system   coupled   with   the   other   ground   support   systems   eliminates   the   need   for   use   of aircraft’s   Auxiliary   Power   Unit,   during   ground   handling,   thereby,   reducing   fuel   costs   and   harmful   emissions   associated   with   the   operation   of   the   APU. However,   the   APU   is   routinely   used   to   start   the   aircraft   engines.   Some   airports   limit   run   time   for   APU   to   a   few   minutes   before   engine   start.   In   the   event   of APU   failure,   additional   power,   usually   portable   GPU,   is   required   to   start   the   engines.   Several   methods   of   providing   ground   power   to   the   gates   can   be utilized: 1.     Portable Diesel fuel powered generator units 2.     Portable solid-state converters 3.     Central ground power system with bank of paralleled rotary generators 4.     Central ground power system with bank of paralleled solid-state converters 5.     Point-Of-Use ground power units Portable equipment frequently used in the small airport, and military field is gradually phase out in the large airports. Central   400   Hz   ground   power   system   is   supply   from   the   bank   of   converters   installed   at   the   strategic   location.   Power   is   typically   distributed   via   575   VAC system   and   converted   to   115/200   V   AC   at   the   gate.   Typically,   3   -   5   frequency   converters,   rotary,   or   static,   electronically   paralleled   are   used   to   share   the   total demand   of   the   system   and   to   provide   redundancy,   should   one   of   the   frequency   converters   fail.   To   achieve   a   desire   voltage   level,   which   should   be   in   the operating   range   of   aircrafts,   a   Line   Drop   Compensator   are   installed   at   the   gate   locations   to   compensate   for   the   inductive   reactance   of   the   distribution network.   The   construction   of   400   Hz   -   575   V   distribution   systems   is   generally   most   costly   due   to   the   need   for   special   distribution   cables   designed   to   reduce excessive voltage drop. Point-Of-Use   (or   Point-Of-Service)   system   involves   conversion   of   power   at   the   point   of   application   near   the   aircraft.   The   60   Hz   fixed   distribution   system   is installed   in   concourse   to   power   solid-state   converters   at   gates.   Solid-state   rectifiers   are   prone   to   generate   3-rd   and   5-th   harmonics   in   the   power   distribution system.    Therefore,    most    manufacturers    equip    them    with    the    harmonics    attenuating    filters.    In    addition    to    further    lower    harmonic    distortion,    some manufacturers are utilizing converters with Insulated Gate Bipolar Transistors instead of SCR. However, IGBT are slightly more expensive. Whether    the    ground    power    system    is    POU    or    central,    the    design    of    each    system    is    unique    and    requires    different    engineering    and    construction considerations. In general, the system should meet recommendation of Mil-Std-704E: -        The voltage at the aircraft receptacle must stay within 113 to 118 V AC. -        The capacity of ground power at the gate shall allow all different type of aircraft programmed for the location, to be served. -        GPS shall be capable to supply aircrafts with No Break Power Transfer. -        Total Harmonic Distortion shall be less than 3%. -        Frequency drift shall be less than 0.05%. -        System shall be operational at to 125 deg. F. -        Acoustical noise shall be below 65 dBA at 1 M distance and 1.5 FT height. -        DC content shall not exceed 100 mV. -        Outdoor housing shall be rated NEMA 3R. -        The mean repair time shall be 20 minutes or less. -        GPS components shall be capable to be connected to BMS for data gathering and the alarm of malfunction. Distribution   of   400   Hz   power   has   inherent   problems   associated   with   high   voltage   drop.   To   counteract   the   problem   a   special   cables   are   being   use   with multiple   (typically   6)   copper   conductors   in   XLPE   insulation,   twisted   as   a   planetary   wrap.   To   assure   bridge   mobility   the   gate   portion   of   fixed   GPS   distribution system   cabling   is   typically   routed   on   the   bridge   roof   or   under   in   expandable   wire-way   system   or   side   mounted   cable   pantograph   system.   In   some   airports Ground   Power   System,   conductors   are   routed   underground   with   the   embedded   service   wells   located   near   the   aircrafts   connection   points.   Older   airports   are often equipped with the central ground power systems powered by 400 Hz banks of vertical, rotary generator strategically located near the concourses.  Concourse Gate Utilization Schedules Several factors should be considered in determination of the most feasible size of the ground power equipment: 1.     The output of the ground electric power equipment installed at each gate shall be adequate to power all type of aircraft, which the gate will serve. 2.     The equipment should be standardized to the high extend to simplify operation, maintenance and limit the variety of spare parts. 3.     Possibility of the future changes in gates assignment should also be considered. 4.     All the above should be couple with the system economics. The ground power requirements, as determined by manufacturers, are usually slightly below total nominal output of the aircraft integral power plant. Configurations of Ground Power System A   variety   of   an   airport   400   Hz   fixed   Ground   Power   System   architecture   could   consider.   The   choice   between   them   depends   on   the   airport   configuration, number   of   gates   to   be   supply,   airport   maintenance   capabilities,   spare   parts   standardization,   etc.   Base   on   the   current   industry   practices   and   future technology   trends,   the   typical   GPS   selection   criteria   below,   have   been   compiled.   However,   these   criteria   are   not   arbitrary   and   should   be   applied   with consideration for the airport geometry, existing GPS, maintenance preference, training cost and projected airport expansion. -                        Decentralized   POU   could   be   considered   for   concourses   with   1-   16   power   delivery   points.   Its   main   advantage   is   the   ease   of   installation,   with   the investment   cost   being   proportional   to   number   of   gates.   Energy   efficiency   with   the   POU   solid-state   converters   is   high   as   the   converters   are   active   only   when aircrafts are connected to it. The same applies to semi-centralized system below. -                        Semi-centralized   system   with   POU   could   be   considered   for   concourses   with   6-16   power   delivery   points.   In   this   solution,   the   same   converters   can supply   more   than   one   gate.   It   is   possible   to   adjust   the   power   system   total   capacity   by   applying   limited   diversity   factor.   In   practice,   this   architecture   depends heavily on the geometry of the concourse, the aircraft-parking schedule and has limited application in the busy airports. -                        For   more   than   16   points   a   centralized   system   could   be   advantageous.   The   total   capacity   and   the   investment   cost   of   the   400   Hz   power   source equipment   could   be   less   if   a   diversity   factor   related   to   number   of   supply   points   is   applied.   On   the   other   hand,   the   cost   of   distribution   cabling   could   be   higher due   to   increased   voltage   droop   associated   with   400   Hz   system.   Should   one   of   the   converters   fail,   the   operation   of   400   Hz   power   supply   central   equipment could   be   maintained,   although   with   limited   capacity.   However,   this   does   not   apply   to   gate   mounted   Line   Droop   Compensators.   Failure   of   the   LDC   would disable   the   power   supply   at   this   gate.   Energy   efficiency   for   centralized   system   is   low   as   the   rotary   400   Hz   generator   must   be   power   constantly.   Some   saving can   be   achieved   by   temporary   halting   one   or   more   generators   depends   on   400   Hz   power   demand.   Additional   factor   to   consider   is   the   cost   of   the   rotary equipment maintenance, the cost of required dedicated floor space and the cost of AC distribution equipment.