05/24/16
Overview
The function of the electrical control system is to control the operation of the aircraft electrical power supplies.
The GCUs give control and protection to the main engine generators and the APU generator. There are five GCUs on the aircraft: one for each main generator and one for the APU generator. The GCUs are installed on the right side of the baggage compartment, above the floor, adjacent to the AC power center (ACPC).
The ELECTRICAL control panel includes the switches and indicators used to operate the electrical supply system. It is installed on the overhead panel in the flight compartment.
05/24/16
Generator Control Units (GCU)
Each GCU gives one generator excitation power, voltage regulation, generator system control and protection, built-in test (BIT), and digital data communication interfaces. The GCUs are interchangeable between the main engine generators and the APU generator.
BIT circuits in each GCU find malfunctions in the GCU and the generator. Power for the operation of the GCU comes from the permanent magnet generator (PMG) of the applicable generator. The GCU gets a 28 VDC back-up supply from the battery bus for the power-up BIT.
The control systems of the GCU are:
- Voltage regulator: The voltage is controlled by the adjustment of the current that flows through the excitation field coils of the generator. This makes sure that the output of the generator does not go above 115 VAC. The voltage regulator includes a circuit that keeps the high phase voltage to 121 VAC in malfunction conditions that are not symmetrical. It also controls the line current during malfunction conditions
- Electrical load: The GCU gives an analog signal to the ACPC for load shedding and restoration to the generator. The signal changes between 0 to 12 VDC in proportion to the generator load current
- Power available signal: The GCU sends a power available signal to the ACPC when the output of the generator is in the specified limits of operation
- Generator line contactor (GLC) control: The GCU energizes the ACPC GLC with 28 VDC when generator power is available and the generator control switch (GCS) is not in the OFF position
The GCU has the protection systems that follow:
- Below voltage
- Over-voltage
- Below frequency
- Over-frequency
- Differential current (feeder fault)
- Over-current
- Overload
- Line current limit during malfunctions
- Line current monitor
- High phase voltage limit during assymmetric malfunctions
- Phase sequence
- Oil over-temperature
- Low oil pressure
- Fire
The GCU built-in-test (BIT) does the checks that follow:
- Operation of the voltage regulation and protection
- Operation of the current protection
- Operation of the frequency sensor
- Operation of the GCU and main generator
The GCU keeps the error data from the BIT in the non-volatile memory (NVM) for subsequent maintenance.
05/24/16
ELECTRICAL Control Panel
The ELECTRICAL control panel is installed on the left side of the overhead panel in the flight compartment. It includes the switches that follow:
- Main GCSs (GEN 1, GEN 2, GEN 3, and GEN 4)
- APU GCS (APU GEN)
- RAT GCS (RAT GEN)
- External AC control switch (EXT AC)
- External DC control switch (EXT DC)
- Battery master switch (BATT MASTER)
The main generator control switches (GEN 1, GEN 2, GEN 3, GEN 4, APU GEN) are used to put the related generator on-line or off-line. These GCSs are single pole, double throw, latched pushbuttons with split dual illuminated indicator.
Each GCS has two positions: on/auto and OFF/reset. The GCS is usually in the on/auto position with the switch released. When the switch is set to the on/auto position, the generator is connected to an aircraft bus. When the switch is pushed to the OFF/reset position, the generator is removed from the aircraft bus, and the logic circuits in the GCU are prepared for reset.
Each GCS has a FAIL (cyan) indicator and an OFF (white) indicator. When the FAIL indicator is on, it shows that the generator is defective. When the OFF indicator is on, it shows that the generator has been turned off.
The RAT GCS (RAT GEN) is a single pole, single throw, momentary pushbutton with split dual illuminated indicator. It puts back primary or auxiliary power to AC bus 4 and AC essential bus if it is selected after a main generator or APU generator reset.
The RAT GEN switch has two positions: normal and override. When the RAT is extended and it supplies the AC essential bus, the ON indicator comes on but the switch is not active. If primary or auxiliary power starts to supply AC bus 4 after a RAT extension, and the manual release lever is retracted, the switch is activated. If the switch is pushed to the override (OFF) position, the RAT GCU de-energizes the RAT line contactor. This disconnects RAT power from the AC essential bus and connects primary or auxiliary power to the AC essential bus through the AC bus 4. In this condition, the RAT generator stays excited (prepared to come on-line) if the switch is pushed again after a malfunction of the primary or auxiliary power supply.
The RAT GEN switch has an ON (green) indicator which is on when the RAT generator is on-line. The switch also has an OFF (white) indicator that is on when the RAT is defective, or when the active switch is pushed to override the RAT. In usual conditions, the two indicators are off.
The EXT AC switch is a single pole, single throw, two position momentary pushbutton. It controls the connection of external AC power to the aircraft for maintenance. The connection of external AC power to the aircraft loads can only be made if the main generators and the APU generator are not on-line.
The switch has an ON (white) indicator and an AVAIL (cyan) indicator. When the ON indicator is on, it shows that the external AC power is on-line to supply the aircraft busses. When the AVAIL indicator is on, it shows that the external AC power is connected to the aircraft (but is not on-line) and is in its correct limits. If the two indicators in the switch are off, the external AC power is not available to supply the aircraft.
The switch has two positions: on and off. The switch is usually in the off position. When the switch is pushed to the on position, the external AC supply is applied to the aircraft to supply all the loads for maintenance. The AVAIL indicator on the EXT AC switch goes off and the ON indicator comes on. When the switch is pushed to the off position, the external AC supply is not applied to the aircraft loads. The AVAIL indicator on the EXT AC switch comes on and the ON indicator goes off.
When the AC ground service mode is selected on the aft service control panel, the EXT AC switch AVAIL indicator stays on and the ON indicator stays off. If the EXT AC switch is pushed to the on position, the AVAIL indicator goes off and the ON indicator comes on. The maintenance mode has priority over the ground service mode.
The EXT DC switch is a single pole, single throw, two position momentary pushbutton. It controls the connection of external DC power to the aircraft for maintenance.
The switch has an ON (white) indicator and an AVAIL (cyan) indicator. When the ON indicator is on, it shows that the external DC power is on-line to supply the aircraft busses. When the AVAIL indicator is on, it shows that the external DC power is connected to the aircraft (but is not on-line) and is within its correct limits. If the two indicators in the switch are off, the external DC power is not available to supply the aircraft.
The switch has two positions: on and off. The switch is usually in the off position. When the switch is pushed to the on position, the external DC supply is applied to the aircraft to supply all the loads. The AVAIL indicator on the EXT DC switch goes off and the ON indicator comes on. When the switch is pushed to the off position, the external DC supply is not applied to the aircraft loads. The AVAIL indicator on the EXT DC switch comes on and the ON indicator goes off.
When the DC ground service mode is selected on the aft service control panel, the EXT DC switch AVAIL indicator stays on and the ON indicator stays off. If the EXT DC switch is pushed to the on position, the AVAIL indicator goes off and the ON indicator comes on. The maintenance mode has priority over the ground service mode, and the AC ground service mode has priority over the DC ground service mode. To select the ground service mode on the aft service control panel, the EXT AC and EXT DC switches must show AVAIL.
The BATT MASTER switch is an eight pole, triple throw, level-lock toggle-switch. It controls the avionics and APU battery power supplies. The three positions of the switch are as follows; OFF, EMS, and ON.
Note:
The connection to the EMS position is made and kept during and after the change from the EMS to the ON position for 4 of the poles. The connection to the OFF position is made and kept during and after the transition from the OFF to the EMS position for the other 4 poles.
When the switch is put in the EMS position with the aircraft on the ground, the available battery power supplies the EMS CDUs only. This lets the electrical-power distribution system be primed and monitored if necessary before power is applied to it. In this position, a ground-type discrete signal is sent to the two EMS CDUs to show that the EMS mode is selected. The EMS mode can be selected with the external AC power connected and available (but not with external AC power on). With the switch in the EMS position, the ACPC gets a 28 VDC lamp supply from the external AC power (if connected) to supply the two EMS CDUs.
When the switch is in the ON position, and there is no transformer rectifier unit (TRU) power, the avionics and APU batteries supply the busses as follows:
- The battery bus when the aircraft is on the ground
- The battery and essential busses when the aircraft is in the air
When the switch is in the ON position, an open ground-type discrete signal is sent to the two EMS CDUs to show that the EMS mode is de-selected.
The BATT MASTER switch is set to the ON position in the aircraft start-up sequence. It stays in that position during the flight until power is shut off at the completion of the mission.
A 28 VDC supply from the avionics battery, through the battery master switch, makes an indicator illuminate on the aft service control panel. The indicator gives a warning to the ground and maintenance crew that the BATT MASTER switch is not in the OFF position.
In the flight compartment, on the EMS CDU, the circuit breakers can be set to OUT with AC external power on and the BATT MASTER switch in the ON position. However, the circuit breakers can be set to LOCK or removed from the LOCK position only if the BATT MASTER switch is in the EMS position (with AC external power not on).
