Power Distribution and Control
The EPDC subsystem distributes
28-volt dc electrical power and generates and distributes 115-volt,
three-phase, 400-hertz ac electrical power to all of the space shuttle
systems' electrical equipment throughout all mission phases. The
EPDC subsystem consists of a three-bus system that distributes electrical
power to the forward, mid-, and aft sections of the orbiter for
equipment used in those areas. The three main dc buses are main
A (MNA), main B (MNB) and main C (MNC). Three ac buses, AC1, AC2
and AC3, supply ac power to the ac loads. Three essential buses,
ESS1BC, ESS2CA and ESS3AB, supply control power to selected flight
crew controls and operational power to electrical loads that are
deemed essential. Nine control buses, CNTL AB1, 2, 3; CNTL BC1,
2, 3; and CNTL CA1, 2, 3, are used only to supply control power
to flight crew controls. Two preflight buses, PREFLT 1 and PREFLT
2, are used only during ground operations.
Electrical power is controlled
and distributed by assemblies. Each assembly-main distribution assembly,
power controller assembly, load controller assembly and motor controller
assembly-is an electrical equipment container or box.
The dc power generated by
each of the fuel cell power plants is supplied to a corresponding
DA. Fuel cell power plant 1 is supplied to DA 1, FCP 2 to DA 2 and
FCP 3 to DA 3. Each DA contains remotely controlled motor-driven
switches called power contactors used for loads larger than 125
amps. The power contactors are rated at 500 amps and control and
distribute dc power to a corresponding mid power controller assembly,
forward power controller assembly and aft power controller assembly.
Power contactors are also located in the APCAs to control and distribute
GSE 28-volt dc power to the orbiter through the T-0 umbilical before
the fuel cell power plants take over the supply of orbiter dc power.
Each of the mid, forward,
and aft PCAs supplies and distributes dc power to a corresponding
mid motor controller assembly, forward motor controller assembly,
forward load controller assembly, aft load controller assembly and
aft motor controller assembly and dc power to activate the corresponding
ac power system.
Each PCA contains remote
power controllers and relays. The RPCs are solid-state switching
devices used for loads requiring current in a range of 3 to 20 amps.
The RPCs are current protected by internal fuses and also have the
capability to limit the output current to a maximum of 150 percent
of rated value for two to three seconds. Within three seconds the
RPC will trip out, which removes the output current. To restore
power to the load, the RPC must be reset, which is accomplished
by cycling a control switch. If multiple control inputs are required
before an RPC is turned on, hybrid drivers are usually used as a
logic switch, which then drives the control input of the RPC.
Each LCA contains hybrid
drivers, which are solid-state switching devices (no mechanical
parts) used as logic switches and for low-power electrical loads
of less than 5 amps. When the drivers are used as a logic switch,
several control inputs are required to turn on a load. Hybrid drivers
are also used in the MPCAs. The hybrid drivers are current protected
by internal fuses. Hybrid relays requiring multiple control inputs
are used to switch three-phase ac power to motors.
Relays are also used for
loads between 20 amps and 135 amps in PCAs and MCAs.
In the midbody there are
no LCAs; therefore, the MPCAs contain RPCs, relays and hybrid drivers.
Each MCA contains main dc buses, ac buses and hybrid relays that
are remotely controlled for control of the application or removal
of ac power to ac motors. The main dc bus is used only to supply
control or logic power to the hybrid relays so the ac power can
be switched on or off.
The remotely controlled
switching devices permit the location of major electrical power
distribution buses close to the major electrical loads, which eliminates
heavy electrical feeders to and from the pressurized crew compartment
display and control panels. In addition, this reduces the amount
of spacecraft wiring, thus weight, and permits more flexible electrical
The No. 1 distribution assembly
and all No. 1 controllers go with fuel cell 1 and MNA bus, all No.
2 controllers and DA 2 go with fuel cell 2 and MNB bus, and all
No. 3 controllers and DA 3 go with fuel cell 3 and MNC bus. The
FC main bus A switch on panel R1 positioned to on connects fuel
cell 1 to the MNA DA and controllers and disconnects fuel cell 1
from the MNA DA and controllers when positioned to off . The talkback
indicator associated with the FC main bus A switch will indicate
on when fuel cell 1 is connected to main bus A DA and controllers
and off when fuel cell 1 is disconnected from main bus A DA and
controllers. The FC main bus B and C switches and talkback indicators
on panel R1 function in the same manner.
Main bus A can be connected
to main bus B or main bus C through the use of the main bus tie
switches on panel R1 and power contactors in the DAs. For example,
main bus A can be connected to main bus B by positioning the main
bus tie A switch to on and the main bus tie B switch to on . The
talkback indicator associated with the main bus tie A and B switches
will indicate on when main bus A is connected to main bus B. To
disconnect main bus A from main bus B, the main bus tie A and B
switches must be positioned to off; the talkback indicators associated
with the main bus tie A and B switches will then indicate off .
Main bus A can be connected to main bus C in a similar manner using
the main bus tie A and C switches. Main bus B can be connected to
main bus A or C in a similar manner using the main bus tie B and
A or C switches. Similarly, main bus C can be connected to main
bus B or A using the main bus C and B or A switches.
Main bus A, B or C voltages
can be displayed on the dc volts meter on panel F9 through the main
volts A, B or C rotary switch on panel F9. The main bus undervolts
red caution and warning light on panel F7 will be illuminated if
main bus A, B or C voltage is 26.4 volts dc, informing the crew
that the minimum equipment operating voltage limit of 24 volts dc
is being approached. A backup caution and warning light will also
be illuminated at 26.4 volts dc. An SM alert light will be illuminated
at 27 volts dc or less, alerting the flight crew to the possibility
of a future low-voltage problem. A fault message also is transmitted
to the CRT.
The nominal fuel cell voltage
is 27.5 to 32.5 volts dc, and the nominal main bus voltage range
is 27 to 32 volts dc, which corre spond to 12- and 2-kilowatt loads,
Depending on the criticality
of orbiter electrical equipment, some electrical loads may receive
redundant power from two or three main buses. If an electrical load
receives power from two or three sources, it is for redundancy only
and not for total power consumption.
Essential buses supply control
power to switches that are necessary to restore power to a failed
main dc or ac bus and to essential electrical power system electrical
loads and switches. In some cases, essential buses are used to power
switching discretes to multiplexers/demultiplexers. Examples of
the selected flight crew switches and loads are the EPS switches,
GPC switches, tactical air navigation mode switches, radar altimeter
and microwave scan beam landing system power switches, the caution
and warning system, emergency lighting, audio control panel, and
master timing unit.
The three essential buses
are ESS1BC, ESS2CA and ESS3AB. ESS1BC receives power from three
redundant sources. It receives dc power from fuel cell 1 through
the ESS bus source FC 1 switch on panel R1 when the switch is positioned
to on and from main dc buses B and C through RPCs when the ESS bus
source MN B/C switch on panel R1 is positioned to on . Electrical
power is then distributed from the essential bus in DA 1 through
fuses to the corresponding controller assemblies and to the flight
and middeck panels. ESS2CA receives power from fuel cell 2 through
the ESS bus source FC 2 switch on panel R1 when positioned to on
and main dc buses C and A through RPCs when the ESS bus source MN
C/A switch on panel R1 is positioned to on. Electrical power is
then distributed from the essential bus in DA 2 through fuses to
the corresponding controller assemblies and to the flight and middeck
panels. ESS3AB receives power from fuel cell 3 through the ESS bus
source FC 3 switch on panel R1 when positioned to on and main dc
buses A and B through RPCs when the ESS bus source MN A/B switch
on panel R1 is positioned to on. Electrical power is then distributed
from the essential bus in DA 3 through fuses to the corresponding
controller assemblies and to the flight and middeck panels.
The ESS bus voltage can
be monitored on the volts meter on panel F9 through the ESS volts
1 BC, 2 CA, 3 AB rotary switch. An SM alert light will be illuminated
to inform the flight crew if the essential bus voltage is less than
25 volts dc. A fault message also is transmitted to the CRT.
Nine control buses are used
to supply only control power to the display and control panel switches
on the flight deck and in the middeck area. A control bus does not
supply operational power to any system loads. The control buses
are enabled by the control bus power MNA, B, C switches on panel
R1 and the MNA control bus BC 1/2/3 circuit breaker on panel R15,
the MNB control bus CA 1/2/3 circuit breaker on panel R15 and the
MNC control bus AB 1/2/3 circuit breaker on panel R15. The corresponding
main bus is connected through RPCs and diodes. Each control bus
receives power from three main dc buses for redundancy. MNA bus
is supplied to three control buses, AB1/2/3, BC1/2/3 and CA1/2/3.
(The numbers 1, 2 and 3 indicate the number of the bus and not a
fuel cell.) MNB bus is supplied to three control buses, AB1/2/3,
BC1/2/3 and CA1/2/3. MNC bus is supplied to three control buses,
AB1/2/3, BC1/2/3 and CA1/2/3. The RPCs are powered continuously
unless the control bus pwr MNA, MNB, MNC switch on panel R1 is positioned
to the momentary reset position, which turns the corresponding RPC's
power off and resets the RPC if it has been tripped off. An SM alert
light is illuminated if the control bus voltage is less than 24.5
volts dc, and a fault message is sent to the CRT. The Mission Control
Center in Houston can monitor the status of each RPC.
Until T minus three minutes
and 30 seconds, power to the orbiter is load shared with the fuel
cells and GSE, even though the fuel cells are on and capable of
supplying power. Main bus power is supplied through the T-0 umbilicals,
MNA through the left-side umbilical and MNB and C through the right-side
umbilical to aft power controllers 4, 5 and 6. From APCs 4, 5 and
6, the GSE power is directed to the DA, where the power is distributed
throughout the vehicle. The power for the PREFLT 1 and PREFLT 2
test buses is also supplied through the T-0 umbilical. These test
buses are scattered throughout the orbiter and are used to support
launch processing system control of critical orbiter loads, although
they also power up the essential buses in the APCs when on GSE.
As in the main bus distribution, essential bus power from the APCs
is directed to the DAs and then distributed throughout the vehicle.
At T minus three minutes 30 seconds, the ground turns off the GSE
power to the main buses, and the fuel cells automatically pick up
the loads. At T minus zero, the T-0 umbilical is disconnected with
the preflight test bus wires live.
Fuel cell 3 may be connected
to the primary payload bus by positioning the pri FC3 switch on
panel R1 to the momentary on position. The talkback indicator next
to this switch will indicate on when fuel cell 3 is connected to
the PRI PL bus. The PRI PL bus is the prime bus for supplying power
to the payloads. Fuel cell 3 may be disconnected from the payload
bus by positioning the pri FC3 switch to the momentary off position.
The talkback indicator will indicate off.
A second source of electrical
power for the PRI PL bus may be supplied from MNB bus by positioning
the pri MN B switch on panel R1 to the momentary on position. The
talkback indicator next to this switch will indicate on. MNB bus
may be removed from the PRI PL bus by positioning the switch momentarily
to off . The talkback indicator will indicate off. A third possible
source of electrical power for the PRI PL bus may be supplied from
MNC bus through the pri MN C switch on panel R1, positioned momentarily
to the on position. The adjacent talkback indicator will indicate
on. MNC bus may be removed from the PRI PL bus by positioning the
switch momentarily to off . The talkback indicator will indicate
There are two additional
payload buses in the aft section of the payload bay at the Yo 1307
aft bulkhead station. The aft payload B bus may be powered up by
positioning the aft MN B switch on panel R1 to on . The aft payload
C bus may be powered up by positioning the aft MN C switch on panel
R1 to on . The off position of each switch removes power from the
corresponding aft payload bus.
The payload aux switch on
panel R1 permits main bus A and main bus B power to be supplied
to the AUX PL A and AUX PL B buses when the switch is positioned
to on. The auxiliary payload buses provide power for emergency equipment
or controls associated with payloads. The off position removes power
from the AUX PL A and PL B buses. It is also noted that the two
auxiliary payload buses may be dioded together to form one bus for
Two or more feeders to the
payload may be used simulta neously, but two orbiter power sources
may not be tied directly within the payload. Any payload equipment
requiring electrical power from two separate orbiter sources is
required to ensure isolation of these power sources so that no single
failure in a load, or succession or propagation of failures in a
load, will cause an out-of-limit condition to exist on the orbiter
system equipment on more than one bus.
The payload cabin switch
on panel R1 provides MNA or MNB power to patch panels located behind
the payload specialist and mission specialist stations located on
the aft flight deck. These patch panels supply power to the payload-related
equipment located on panels at these stations. Two three-phase circuit
breakers, AC2 cabin PL3 J and AC3 cabin PL3J, on panel MA73C provide
ac power to the payload patch panels.
is generated and made available to system loads by the EPDC subsystem,
using three independent ac buses, AC1, AC2 and AC3. The ac power
system includes the ac inverters for dc conversion to ac and inverter
distribution and control assemblies containing the ac buses and
the ac bus sensors. The ac power is distributed from the IDCAs to
the flight and middeck display and control panels and from the MCAs
to the three-phase motor loads.
Each ac bus consists of
three separate phases connected in a three-phase array. Static inverters,
one for each phase, are located in the forward avionics bays. Each
inverter has an output voltage of 116 to 120 volts root mean square
at 400 hertz, plus or minus 7 hertz.
The inverters are controlled
by the inv pwr 1, 2, 3 switches on panel R1. Inverter 1 receives
power only from MNA, inverter 2 from MNB and inverter 3 from MNC.
All three inverters of inverter 1 receive MNA bus power when the
switch is positioned to on , and all three must be in operation
before the adjacent talkback indicator indicates on . The indicator
will show off when main bus power is not connected to the inverter.
The inv/ac bus 1, 2, 3 switches
on panel R1 are used to apply each inverter's output to its respective
ac bus. An indicator next to each switch shows its status, and all
three inverters must be connected to their respective ac buses before
the indicator shows on . The talkback indicator will show off when
the three inverters are not connected to their respective ac bus.
The inv pwr and inv/ac bus
switches must have control power from the ac contr circuit breakers
on panel R1 in order to operate. Once ac power has been established,
these circuit breakers are opened to prevent any inadvertent disconnection,
whether by switch failure or accidental movement of the inv pwr
or inv/ac bus switches.
Each ac bus has a sensor,
switch and circuit breaker for flight crew control. The AC1, 2,
3 snsr circuit breakers located on panel O13 apply essential bus
power to their respective ac bus snsr 1, 2, 3 switch on panel R1
and operational power to the respective inv/ac bus switch indicator.
The ac bus snsr 1, 2, 3 switch selects the mode of operation of
the ac bus sensor: auto trip, monitor or off . The ac bus sensor
monitors each ac phase bus for over- or under voltage and each phase
inverter for an overload signal. The overvoltage limits are bus
voltages greater than 123 to 127 volts ac for 50 to 90 milliseconds.
The undervoltage limits are bus voltages less than 102 to 108 volts
ac for 6.5 to 8.5 milliseconds. An overload occurs when any ac phase
current is greater than 14.5 amps for 10 to 20 seconds or is greater
than 17.3 to 21.1 amps for four to six seconds.
When the respective ac bus
snsr switch is positioned to the auto trip position and an overload
or overvoltage condition exists, the ac bus sensor will illuminate
the respective yellow ac voltage or ac overload caution and warning
light on panel F7 and trip out (disconnect) the inverter from its
respective phase bus for the bus/inverter causing the problem. There
is only one ac voltage and one ac overload caution and warning light;
as a result, all nine inverters/ac phase buses can illuminate the
lights. The ac volts meter and rotary switches ( AC1 JA, JB, JC;
AC2 JA, JB, JC; AC3 JA, JB, JC) on panel F9 or the CRT display would
be used to determine which inverter or phase bus caused the light
to illuminate. The phase bus causing the problem would show zero
volts. Because of the various three-phase motors throughout the
vehicle, there will be a small induced voltage on the phase bus
if there is only one phase that has loss of power.
Before power can be restored
to the tripped bus, the trip signal to the inv/ac bus switch must
be removed by positioning the ac bus snsr switch to off , then back
to the auto trip position, which extinguishes the caution and warning
light. The inv/ac bus switch is then positioned to on, restoring
power to the failed bus. If the problem is still present, the sequence
will be repeated.
If an undervoltage exists,
the yellow ac voltage caution and warning light on panel F7 will
be illuminated, but the inverter will not be tripped out from its
When the ac bus snsr 1,
2, 3 switches are in the monitor position, the ac bus sensor will
monitor for an overload, overvoltage and undervoltage and illuminate
the applicable caution and warning light; but it will not trip out
the phase bus/inverter causing the problem.
When the ac bus snsr switches
are off, the ac bus sensors are non-operational, and all caution
and warning and trip-out capabilities are inhibited.
A backup caution and warning
light will be illuminated for overload or over- and undervoltage
conditions. The SM alert will occur for over- and undervoltage conditions.
A fault message also is sent to the CRT.
There are 10 motor controller
assemblies used on the orbiter: three are in the forward area, four
are in the midbody area, and three are in the aft area. Panel MA73C
contains the controls for the MCAs. Their only function is to supply
ac power to non-continuous ac loads for ac motors used for vent
doors, air data doors, star tracker doors, payload bay doors, payload
bay latches and reaction control system/orbital maneuvering system
motor-actuated valves. The MCAs contain main buses, ac buses and
hybrid relays, which are the remote switching devices for switching
the ac power to electrical loads. The main buses are used only to
supply control or logic power to the hybrid relays so that ac power
can be switched on and off. If a main bus is lost, the hybrid relays
using that main bus will not operate. In some cases, the hybrid
relays will use logic power from a switch instead of the MCA bus.
The three forward motor
controller assemblies (FMC 1, FMC 2 and FMC 3) correspond to MNA/AC1,
MNB/AC2 and MNC/AC3, respectively. Each FMC contains a main bus,
an ac bus and an RCS ac bus. The main bus supplies control or logic
power to the relays associated with both the ac bus and RCS ac bus.
The ac bus supplies power to the forward left and right vent doors,
the star tracker Y and Z doors, and the air data left and right
doors. The RCS ac bus supplies power to the forward RCS manifold
and tank isolation valves.
The aft motor controller
assemblies (AMC 1, AMC 2 and AMC 3) correspond to MNA/AC1, MNB/AC2,
and MNC/AC3, respectively. Each AMC assembly contains a main bus
and its corresponding ac bus and a main RCS/OMS bus and its corresponding
RCS/OMS ac bus. Both main buses are used for control or logic power
for the hybrid relays. The ac bus is used by the aft RCS/OMS manifold
and tank isolation and crossfeed valves.
The mid motor controller
assemblies (MMC 1, MMC 2, MMC 3 and MMC 4) contain two main dc buses
and two corre sponding ac buses. MMC 1 contains main bus A and B
and their corresponding buses, AC1 and 2. MMC 2 contains MNB and
AC2 and AC3 buses. MMC 3 contains the same buses as MMC 1, and MMC
4 the same buses as MMC 2. Loads for the main buses/ac buses are
vent doors, payload bay doors and latches, radiator panel deployment
actuator and latches, and payload retention latches.
The electrical components
in the midbody are mounted on cold plates and cooled by the Freon-21
system coolant loops. The PCAs, LCAs, MCAs and inverters located
in forward avionics bays 1, 2 and 3 are mounted on cold plates and
cooled by the water coolant loops. The inverter distribution assemblies
in forward avionics bays 1, 2 and 3 are air-cooled. The LCAs, PCAs
and MCAs located in the aft avionics bays are mounted on cold plates
and cooled by the Freon-21 system coolant loops.