Electrical power systems

Electricity is the nervous system of a modern aircraft. It powers the avionics, the displays, the lights and the radios, and on newer designs it has taken over work once done by hydraulics and engine bleed air. On the Tern Block 2 it does far more: the propellers themselves are driven by electric motors, so the electrical system carries hundreds of kilowatts of propulsive power as well as the few kilowatts that run the rest of the aircraft. This lesson covers how aircraft electrical systems are organised, how engineers size them with an electrical load analysis, how they are protected, and what changes when the voltage rises from 28 V to the 800 V of the Tern's propulsion bus.

Sources, buses and architectures

Small and medium aircraft have traditionally used 28 V direct current. Each engine drives a starter-generator, a machine that turns the engine to start it and then generates power once the engine is running, and a battery supports engine starts and emergencies. Larger aircraft use 115 V alternating current, either at a constant 400 Hz from a generator with a constant-speed drive or, in newer designs, at a variable frequency between about 360 and 800 Hz from a generator driven directly by the engine. Some recent airliners add 270 V or ±270 V direct current for large electric loads such as cabin compressors.

Power is distributed through buses: conductors that collect power from sources and feed groups of loads through circuit breakers or solid-state power controllers. The architecture arranges buses so that the most important loads can always be fed. A typical scheme has main buses fed by the generators, essential buses that can be fed from either main bus or from the battery, and an emergency or hot battery bus connected directly to the battery for the few loads that must work in any situation. Bus tie contactors can connect the left and right sides if one generator fails, and automatic load shedding drops non-essential loads such as galley ovens or cabin entertainment when generating capacity is lost.

The Swift 900 study uses a conventional architecture: two 28 V starter-generators rated at 400 A each, one on each engine, a main lithium-ion battery, a standby battery for the emergency bus and a static inverter for the few loads that need alternating current. If both generators fail, the batteries must run the essential loads long enough to descend and land. Many standards, including the rules for business jets, express this as a duration, commonly 30 minutes for essential loads after loss of all normal generation, though the actual requirement depends on the aircraft's certification basis and its operating rules.

The Tern's two-voltage system

The Tern Block 2 has two electrical worlds. The high-voltage propulsion system runs at about 800 V direct current: the turboshaft-driven generator feeds a rectifier, the two battery strings feed through contactors, and the power distribution unit supplies the two 450 kW propeller motor inverters. The aircraft systems run from a conventional 28 V bus, fed by two 4 kW DC-DC converters that step the high voltage down, plus a dedicated 28 V emergency battery that is electrically separate from the propulsion batteries. The separation is deliberate: a fault in the high-voltage system must not be able to take the avionics, the flight controls or the displays with it.

Why 800 V? Power is voltage times current, so for the 900 kW that both motors can draw at take-off, 800 V needs about 1100 A while 270 V would need more than 3300 A. Cable mass and resistive losses fall roughly with the current, so higher voltage saves weight. The price is insulation: high voltage at altitude is much harder to contain than at sea level, and the protection, inspection and maintenance training must be designed for it.

The electrical load analysis

The electrical load analysis, or ELA, is the basic sizing document for any aircraft electrical system and a required part of the certification data. It lists every load, its power, and how much of each flight phase it runs. Summing by phase gives the demand on the sources, which must be within their continuous rating with margin; summing the essential loads gives the battery size for emergencies. The ELA is revised every time a load is added, which on a long programme means many times, and it is one of the documents a new systems engineer is most likely to be handed on the first day.

Arjun's ELA for the Tern's 28 V bus groups the loads into eight lines. Each has a nameplate power and a duty factor for each of four phases: ground operation, take-off and climb, cruise and the emergency case in which both converters have failed and the 28 V emergency battery must carry the aircraft to a landing.

# Electrical load analysis for the Tern Block 2 28 V aircraft bus, the emergency battery size,
# and the efficiency chain from the turboshaft to the propeller shafts. Loads are illustrative.
loads = {  # name: (watts, fraction of time on in each phase: ground, take-off/climb, cruise, emergency)
    'avionics and displays':        (900, (1, 1, 1, 1)),
    'flight control actuators':     (650, (0.2, 0.6, 0.3, 0.5)),
    'propulsion controllers (28 V)': (420, (1, 1, 1, 1)),
    'battery cooling pumps':        (380, (0.5, 1, 0.7, 1)),
    'windshield heat':              (1500, (0, 0.5, 0.5, 0)),
    'lighting':                     (350, (1, 1, 0.4, 0.3)),
    'cabin fans and heater control': (600, (1, 0.8, 0.8, 0)),
    'fuel pumps (turboshaft)':      (300, (1, 1, 1, 1)),
}
phases = ('ground', 'take-off/climb', 'cruise', 'emergency')
cap = 2 * 4000.0   # two 28 V DC-DC converters from the high-voltage bus, 4 kW each
for i, ph in enumerate(phases):
    tot = sum(w * f[i] for w, f in loads.values())
    note = ''
    if ph != 'emergency':
        note = f"  {100*tot/cap:4.0f} % of both converters, {100*tot/(cap/2):4.0f} % of one"
    print(f"{ph:15s} {tot:6.0f} W{note}")

# Emergency battery: essential loads for 30 minutes after loss of both converters
P_em = sum(w * f[3] for w, f in loads.values())
t_em, V, dod, ageing = 0.5, 25.0, 0.8, 0.8      # hours, average volts, usable depth, end-of-life capacity
Ah = P_em * t_em / V / dod / ageing
print(f"\nemergency load {P_em:.0f} W for 30 min -> battery about {Ah:.0f} Ah at 28 V nominal")

# High-voltage chain efficiency, turboshaft shaft to propeller shaft
chain = {'generator': 0.96, 'rectifier and power distribution unit': 0.985,
         'cables': 0.99, 'motor inverter': 0.98, 'motor': 0.955}
eff = 1.0
for k, e in chain.items():
    eff *= e
    print(f"after {k:38s} {eff:.3f}")
P_shaft = 571.0
print(f"turboshaft must deliver {P_shaft/eff:.0f} kW for {P_shaft:.0f} kW at the propellers; "
      f"heat to remove {P_shaft/eff - P_shaft:.0f} kW")

ground            2890 W    36 % of both converters,   72 % of one
take-off/climb    3970 W    50 % of both converters,   99 % of one
cruise            3451 W    43 % of both converters,   86 % of one
emergency         2430 W

emergency load 2430 W for 30 min -> battery about 76 Ah at 28 V nominal
after generator                              0.960
after rectifier and power distribution unit  0.946
after cables                                 0.936
after motor inverter                         0.917
after motor                                  0.876
turboshaft must deliver 652 kW for 571 kW at the propellers; heat to remove 81 kW

The demand peaks in take-off and climb at about 3970 W, half the capacity of both converters. With both converters working the margin is generous, but the design rule is that the aircraft must be safe after any single failure, and with one converter failed the take-off load is 99 percent of the survivor's rating. That is not acceptable as a continuous condition, so the design adds automatic load shedding: if one converter fails, the windshield heat and part of the cabin loads drop off, bringing the demand back to about 70 percent of one converter. Arjun records the shedding logic in the ELA and in the system requirements so that the test team knows to verify it.

In the emergency case the essential loads total about 2.4 kW. For 30 minutes at an average battery voltage of 25 V, allowing 80 percent usable depth of discharge and 80 percent of rated capacity at the end of the battery's life, the battery needs about 76 Ah. That is a heavy battery for a small aircraft, and Arjun's next task is to see which emergency loads can be reduced. The battery cooling pumps are in the list because the propulsion batteries must be kept within their temperature limits even when they are not being used, and the team decides to let the propulsion system power its own cooling through the distribution unit whenever the high-voltage system is available.

Converters, motors and efficiency

Every conversion stage loses a little energy as heat, and on a hybrid aircraft those small losses add up to a large heat load. The program follows the chain from the turboshaft's shaft to the propellers' shafts: generator, rectifier and distribution unit, cables, motor inverter, motor. With efficiencies between 95.5 and 99 percent each, the chain delivers 87.6 percent of the shaft power. For the 571 kW the propellers need at cruise, the turboshaft must provide 652 kW, the same figure Dr. Nakamura's propulsion group found in their analysis, and 81 kW of heat must be carried away by the cooling system. That is a larger heat load than the entire cabin and avionics cooling of many business jets, and it shapes the Tern's nacelle design.

Power electronics are now built mainly from silicon carbide transistors, which switch faster and run hotter than silicon, saving mass and cooling. Fast switching brings its own problems: high rates of voltage change stress motor insulation and generate electromagnetic interference that must be filtered so it does not reach the radios and navigation receivers.

Protection and wiring

Wires must be protected so that a fault cannot overheat them. Thermal circuit breakers have served for decades; solid-state power controllers now do the same job electronically, with programmable trip curves and the ability to report their status. Arc fault circuit breakers detect the erratic current of an electrical arc, which may be too small to trip an ordinary breaker but hot enough to start a fire in a wire bundle. The 1998 loss of a wide-body airliner off Nova Scotia, traced to arcing in wiring above the cockpit and flammable insulation blankets, led to major changes in wiring inspection and material flammability rules on both sides of the border.

Wiring itself is a system. The industry approach known as electrical wiring interconnection systems, or EWIS, treats the wires, connectors, clamps and routing as equipment that needs design, safety analysis and maintenance like any other. Redundant channels must be routed apart so that a single chafe, fire or bird strike cannot cut both. On the Tern, the two battery strings' harnesses run down opposite sides of the fuselage for exactly that reason, which supports the independence claimed in Lesson 1's fault tree.

High voltage at altitude

Air insulates less well as pressure falls. Paschen's law describes how the breakdown voltage of a gas gap depends on the product of pressure and gap distance, and for typical small gaps the breakdown voltage at the pressures found at cruise altitude is much lower than at sea level. Below full breakdown, partial discharge can occur: small discharges in voids within insulation or at sharp edges, which slowly erode the insulation until it fails. Voltages that are routine on the ground can therefore be dangerous in an unpressurized nacelle at 6000 m.

The Tern's high-voltage equipment is designed and tested for this. Connectors have long creepage paths and are sealed; cables use insulation systems qualified for partial discharge; and the equipment is tested in an altitude chamber at the lowest pressure it will see with margin. Marcus Webb's structures group has also had to learn something new: the battery boxes are pressure-tight enclosures with venting paths designed so that thermal runaway gases, if they occur, are carried overboard rather than into the cabin. Containment of battery thermal runaway is one of the special conditions Transport Canada has applied to the Tern.