13 AC Electrics -Practical Aircraft Systems
Parallel Bus Bar System
Systems Aircraft Practical - Electrics AC 13
Figure 13.5 illustrates a four generator paralleled system. This system allows various combinations of alternator operation. Operation of the system begins with the excitation of the alternator field which will bring its output within the limits required before operation of the GCB can occur.
When the GCB closes it connects its associated alternator to its Load Bus Bar. Once the GCB has closed it will remain closed during all normal circuit functioning.
The Bus Tie Breakers are normally closed so that the closure of the GCB effectively connects the alternator to the Synchronizing Bus Bar. If the other one of a pair of alternators (1 & 2) or (3 & 4) now comes “on line” it too will be joined in parallel to the synchronizing bus bar, but only once the voltage, frequency and phase sequence have been satisfied allowing its GCB to close.
In the system described there are two synchronizing bus bars which can be combined or isolated by the Split System Breaker (SSB) depending on the flight phase or other system requirement. Keeping the synchronizing bus bars isolated from each other will allow the alternators to operate as two paralleled pairs which would be a requirement for example during a dual autopilot autoland to enable the two autopilots to have totally separate power supplies.
If a single alternator failure occurs with a system similar to that shown in Figure 13.5, then opening of the associated GCB will allow its paired alternator to feed the loads of both of them. However, this would place a larger load upon that alternator than is being carried by the pair on the other synchronizing bus bar.
Closure of the SSB would bring all three alternators into parallel operation, thus sharing the total aircraft load between them. Failures are not always that simple however. If there was an earth fault on a load bus bar for instance, opening of the associated GCB would do little to help, the other alternator/s would now be attempting to feed the earth fault. Operation of the BTB associated with the faulty bus bar would prevent the serviceable alternators being affected by the fault, and then the earth fault could be totally isolated by opening the GCB of the alternator feeding it.
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SYNCHRONIZING BUS |
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SYNCHRONIZING BUS |
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Figure 13.5 Parallel alternator operation
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AC Electrics - Practical Aircraft Systems 13
An example of an aircraft with this type of paralleled system is the Boeing 747 - 400. Shown below in Figure 13.6 and Figure 13.7 are the control panel and the EICAS display for the electrical system.
Figure 13.6 747 - 400 electrical control panel
Figure 13.7 747 - 400 EICAS electrical display
AC Electrics - Practical Aircraft Systems 13
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13 Questions
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1. |
The purpose of a synchronizing bus bar is to: |
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a. |
enable interconnections to be made between generator bus bars |
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b. |
supply essential services |
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c. |
monitor on load currents |
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d. |
interconnect DC bus bars |
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2. |
Fuses and circuit breakers are fitted: |
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a. |
in DC circuits only |
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b. |
in both AC and DC circuits |
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c. |
in AC circuits only |
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d. |
only to protect the wiring |
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3. |
Where the aircraft’s main electrical supply is AC, DC requirements are met by: |
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a. |
batteries |
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b. |
TRUs |
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c. |
inverters |
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d. |
a static inverter |
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4. |
In a split bus system using non-paralleled constant frequency alternators as the |
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primary power source: |
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a. |
essential AC loads are supplied directly from No. 1 AC bus bar |
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b. |
essential AC loads are supplied directly from No. 2 AC bus bar |
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c. |
only non-essential AC loads are supplied from the AC bus bars |
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d. |
essential AC loads are normally supplied from No. 1 AC bus bar via the |
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changeover relay |
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5. |
In a split bus system using non-paralleled constant frequency alternators as the |
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primary power source, if both alternators fail: |
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a. |
all non-essential services are lost |
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b. |
all non-essential services will be supplied direct from the battery bus bar |
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c. |
all non-essential services will be supplied from the static inverter |
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d. |
essential DC consumers only will be supplied from the No. 1 TRU, all other DC |
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services will be lost |
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6. |
In normal operation, the split bus bar AC system takes its DC supply from: |
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a. |
two TRUs which are always isolated |
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b. |
a battery which is supplied from No. 1 TRU only |
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c. |
two TRUs which are connected together by the isolation relay |
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d. |
the static inverter |
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7. |
The static inverter in the split bus system supplies: |
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a. |
the essential DC consumers |
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b. |
the essential AC consumers |
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c. |
both essential and non-essential consumers |
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d. |
the batteries |
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8. |
In the split bus system, the AC bus bars: |
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are automatically connected via the isolation relay if one alternator fails |
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b. |
are automatically connected via the bus tie breaker if one alternator fails |
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c. |
can be connected together by switch selection if one alternator fails |
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d. |
can never be connected together because there is no load sharing circuit |
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9. |
With parallel generator operation: |
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a. |
there are two synchronizing bus bars which are normally kept isolated |
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b. |
the GCBs connect the generators to the synchronizing bus bar |
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c. |
the BTBs connect the synchronizing bus bars together |
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d. |
the GCRs connect the generators to their load bus bars |
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10. |
In a parallel alternator operation, should one alternator fail, then: |
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a. |
the other alternators can be selected to supply its load |
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b. |
the failed alternator’s loads will not be supplied |
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c. |
the GCB of the failed alternator will remain closed to allow its loads to be |
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supplied by the remaining alternators |
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d. |
the SSB will close allowing the three remaining alternators to share all of the |
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load |
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11. |
If external power is plugged into an aircraft which utilizes the split bus system of |
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power distribution, then: |
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b. |
it will only supply non-essential AC consumers |
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a. |
it will automatically parallel itself with any alternators already on line |
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c. |
it will supply all the aircraft services |
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d. |
essential AC consumers will be supplied from the static inverter |
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