Boeing 707 Electrical Power

The aircraft’s electrical system is supplied by four AC generators, with one generator driven by each engine. Each unit is rated at 40 kVA and provides up to 36 kW of continuous power at 115/200 volts and 400 Hz.
The generators are controlled from the flight engineer’s electrical panel. Each generator is operated through three control relays, with an amber indicator light showing when the associated relays are open. The relay circuits also incorporate automatic trip functions for generator protection.
To maintain a stable 400 Hz electrical output, each generator is driven at approximately 6,000 RPM through a constant-speed drive, regardless of engine speed.

CONSTANT SPEED DRIVE (CSD)
The Flight Engineer’s electrical panel provides the main controls and indications for the Constant Speed Drives (CSDs). These include a frequency adjustment control, frequency and oil-temperature gauges, a low-oil-pressure warning light, and a CSD disconnect switch.
When a generator is operating independently, the frequency control can be used to fine-tune the generator frequency over a range of approximately 12 Hz. This adjustment is intended for isolated operation and should not be used while generators are operating in parallel.
The CSD low-oil-pressure warning lights normally illuminate before engine start. They also provide a useful indication that the CSD has begun turning: as engine N2 increases to approximately 15–20%, the corresponding warning light should extinguish.
Selecting the CSD disconnect switch activates a solenoid within the disconnect mechanism. This mechanically separates the CSD from the engine accessory drive and simultaneously opens the associated main generator relay.
Once disconnected, a CSD cannot be reconnected during flight. It can only be reset after landing, on the ground.
Following disconnection, the CSD oil temperature may rise and remain elevated until the engine is shut down. This is caused by heat from the engine compressor section combined with the loss of normal oil circulation through the disconnected CSD.

LOAD CONTROLLER
Each load controller performs two primary tasks within the electrical system.
First, it regulates the CSD output speed at approximately 6,000 RPM by controlling the signal sent to the flyweight governor incorporated in the CSD.
Its second function is to manage the electrical load shared by the generators, ensuring that the kW load is distributed and kept balanced between the individual CSDs and generators.
If a bus tie or main generator relay is opened, the kW load-sharing signals are interrupted. The load controller will nevertheless continue to regulate CSD speed.
There are four load controllers, all installed in the electrical rack located in the E&E compartment.

GENERATOR CONTROL UNIT
Each Generator Control Unit (GCU) is responsible for controlling its associated generator and providing protection against faults within the generator system.
The aircraft is equipped with four GCUs, installed in the electrical rack in the lower E&E compartment.
During engine operation, each GCU normally receives power from two independent sources: its associated generator and an essential DC supply fed through a circuit breaker on panel P-6. If one of these sources is lost, the second source remains available to maintain GCU operation.
The two DC supplies are routed through a common circuit breaker located on the front of the GCU. From there, the control circuits extend to the cockpit switches for the corresponding GFR, MGR and BTR relays, together with their associated indicator lights.
With the engines shut down while the aircraft is on the ground, the relay control circuits and their indicator lights can be powered either by the aircraft battery or by an external power source.

GENERATOR CONTROL SWITCH / aka GENERATOR FIELD RELAY (GFR)

The Generator Field Relay (GFR) is a small relay installed within the Generator Control Unit (GCU). When energized and closed, it connects the DC excitation supply from the voltage regulator to the generator field windings.
The relay incorporates a self-latching function and normally remains closed even after the associated engine has been shut down. It can be opened either by a fault signal from the GCU or by selecting the corresponding control switch to the open position. When the relay is open, the FIELD OPEN indicator illuminates.
Opening the GFR also causes the associated main generator breaker to trip.
If the GFR is open while the Essential AC Bus is supplied from that generator, the Essential AC supply will be lost.
Activation of an engine fire control also causes the generator field and main generator relays to open after a time delay of approximately 5–10 seconds.

 

GENERATOR BREAKER

The Generator Breaker connects the generator’s 200-volt AC output to its corresponding AC bus when closed.
The breaker is interlocked with the Generator Field Relay and cannot close until the field relay is energized and the generator has reached an acceptable frequency and voltage.
After engine start, closing the first Generator Breaker automatically removes external power and trips the master switches for the galley and air-conditioning systems.
The remaining generator breakers are brought online automatically through automatic paralleling signals from the Generator Control Units (GCUs).
Under normal conditions, the generator breaker opens automatically when the associated engine is shut down.
It can also be tripped open by either selecting external power or operating the CSD disconnect switch.

 

BUS TIE BREAKER

 

The Bus Tie Breaker (BTB) connects an individual AC bus to the common Sync Bus. With all four BTBs closed, any AC bus can be supplied by any of the aircraft’s generators.
When multiple generators are operating, the closed bus ties allow the electrical load to be shared evenly between the available generators.
Under normal conditions, the BTBs remain closed both on the ground and in flight.
If a fault occurs on the Sync Bus and all BTBs open, the normal and essential AC buses will generally remain powered by their respective generators. However, a generator may subsequently trip offline if an overload condition occurs.
Operating with the bus ties open is undesirable because a generator failure can result in the loss of the associated AC bus, while the remaining generators may also become subject to increased loading.
When the aircraft is on external power and all BTBs are open, the AC buses will not receive power.
CAUTION: Do not close a BTB while the associated Generator Breaker (GB) is closed unless the AC Paralleling Selector Switch is selected to the corresponding generator and the synchronizing lights are extinguished.

SYNCHRONIZING LIGHTS

The synchronizing lights are connected through the AC Paralleling Selector Switch to the appropriate phases of the selected power source and the synchronizing bus-tie circuit.
Their indication provides information about the phase, voltage and frequency relationship between the selected generator and the synchronizing bus. Depending on the difference between the two sources, the lights may flash together or remain continuously illuminated.
The lights are used during manual generator paralleling and also provide a means of checking the phase rotation of a generator or external power source.
If the lights alternate rather than flashing together, this indicates incorrect phase rotation. The generator breaker or external power contactor should not be closed until the phase relationship has been corrected.
During normal generator paralleling, the generator breakers are closed with the bus tie breakers already closed. If a generator breaker does not close, its associated BTB should first be opened. The generator breaker can then be closed, followed by closing the BTB once the synchronizing lights have gone out.
If necessary, generator frequency can be adjusted to reduce the rate at which the synchronizing lights flash, allowing the sources to approach synchronization more slowly.

 

ESSENTIAL POWER SELECTOR SWITCH

The Essential AC Bus receives its power directly from the generator selected as the essential power source. Manually opening the associated Generator Breaker (GB) does not remove power from the bus.
An automatic opening of the GB, however, will interrupt the Essential AC supply. This can occur when the CSD speed switch operates or when the associated Generator Control Relay trips.
The Essential AC Bus can only be powered from an external source when the external power contactor is closed.
Generator No. 2 is normally not selected as the source for essential power. If BTB No. 2 were to open, Generator No. 2 would then be required to supply both the Essential Radio and Instrument Bus and the No. 2 Radio and Instrument Bus.

 

CLOSING A GENERATOR BREAKER RELAY
Closing the first Generator Breaker (GB) after engine start initiates a sequence of automatic electrical switching operations.
The CSD speed switch must first operate, completing the control circuit for the Essential Power Selector and the Main Generator Relay switch. When the first GB switch is moved to the CLOSE position, Auxiliary Negative Sequence Relay No. 2, located in the main AC power shield, is energized. This causes the external power, galley power, and air-conditioning master switches to trip off.
Disconnecting these loads and removing external power prevents the first generator from being subjected to an excessive load when it is connected to the aircraft electrical system.
As the GB switch is held in the CLOSE position, the external power interlock relay operates immediately after the external power switch has tripped. The interlock circuit verifies that the external power main and auxiliary relays have actually opened and have not remained welded closed.
Once the external power relays are confirmed open, the interlock relay closes and supplies power to the Generator Control Unit (GCU). If the GCU detects that both the Sync Bus and the associated AC bus are unpowered, it generates the command required to close the Generator Breaker.
With the bus tie relays closed, closing the first Generator Breaker allows that generator to supply power to all four AC buses.