A Design for a Dream

A look at the GPU design challenge posed by the Boeing 787 Dreamliner


This past December, Boeing celebrated its long-awaited initial test flight of the 787 Dreamliner. It also was a special day for INET Airport Systems as Boeing chose to utilize an INET mobile diesel-powered 180KW/225Kva ground power unit (MDG5-225-C) to provide 400 Hz power to the aircraft when it had landed and was parked for their press conference.

In addition, this benchmarked a successful conclusion to the development program for the MDG5, 180 KW or 225Kva GPU, which started in November 2006. On that date, INET was notified of the award of eight units to be designed, engineered and manufactured for use on this special aircraft.

SPECIFICATION CHALLENGES
The design phase and eventual engineering of the first articles went through a “baptism of fire,” so to speak, as this was the first mobile GPU of this size and capacity and certainly the first of its type and kind for this particular aircraft.

The following design specifications presented the INET engineering staff with numerous challenges:

  • NEMA 3R, steel enclosure with specified Boeing dimensions
  • 400 horsepower Tier 3 engine
  • 150 percent overload capacity
  • Very low noise (< 78 dbA at 3 meters)
  • Channeled air-flow design
  • State-of-the-art controls with digital data reporting capability stored in nonvolatile memory with data logger option for review and printing of last 200 events.
  • High power rating of 180Kva at 1.0 power factor (unity)
  • Tight electrical performance
  • Dual output cables

THE DESIGN PHASE – HIGHLIGHTS
After many hours of producing sketches then drawings, contacting various specialized vendors, and making numerous drawing revisions — which had to be approved by the Boeing engineers — a basic framework of the conceptual design began to take place.

Some of the key design highlights that were eventually engineered into the unit were:

  • Wide access doors with good accessibility to all service points
  • High flow cooling with sound attenuated design
  • Protected stainless-steel fuel tank with a capacity of 135 gallons for 8 -10 hours minimum operation
  • Ruggedized frame design with sprung rear running gear and fork pockets all with specific size dimensions
  • Modern electronic controls with fault reporting
  • Front towbar activated brakes
  • Low fuel warning with shutdown
  • Cold start with glow plugs and block heater

THE POWER QUALITY PROFILE AND CHARACTERISTICS
The following are a few of the power characteristics of the MDG5 unit:

  • Voltage of 115/200 WYE continuous
  • Meets or exceeds Mil-Std-704F for transient voltage recovery time with full recovery to within 1% nominal 100 ms (no load to rated load)
  • Does not exceed 0.5 percent steady state voltage
  • Has automatic line drop compensation of up to 15 percent
  • Voltage adjustment of + 15 volts
  • Harmonics of 1.5 percent max L-L
  • The crest factor is 1.4 + 0.07
  • Phase voltage balance does not exceed 2 percent with 1/3 rated current on unbalanced load
  • The frequency is 400Hz steady state at + 2hz
  • An overload of 125 percent rated load for 5 minutes and 150 percent for 10 seconds.

THE CONTROL PANEL INSTRUMENTATION/DISPLAYS
In addition to those listed above the engine meters:

  • System voltage
  • Coolant temperature
  • Fuel level
  • Oil pressure
  • Faults and the digital generator outputs meter
  • AC voltage and current
  • Ammeter
  • Volts
  • Current

THE FINISHED PRODUCT
The first two units were completed and available for inspection by the Boeing representatives in October 2007 with the following features:

  • Cummins QSM Tier III engine
  • Digital ECM engine controller
  • Low oil pressure shutdown
  • 24VDC system (12V available)
  • High coolant temperature shutdown
  • Cold weather grid heater
  • Meets all power requirements for not only the 787, but for all the latest commercial and military aircraft to include missile and radar equipment, plus AWAC aircraft

787 GROUND POWER REQUIREMENTS
The new Dreamliner utilizes two forward ground power receptacles and one mid-aft ground power receptacle. Each receptacle is rated at 90Kva, and is standard on all Boeing current production widebody airplanes. By design, each receptacle can support continuous 90 Kva and a peak power consumption of 112kva for up to five minutes regardless of the output capacity of the ground power unit. For continuous supply, the airplane will draw less than 90Kva from any receptacle.

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