(2)  
Protection is provided from overvoltage, undervoltage, overexcitation and underexcitation conditions. The unit also provides differential protection, exciter ceiling protection and (except with transistorized voltage regulators) stability protection.

500 
24-21-0 Page 10  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Aug 15/69 

C.  In addition to the control and protective functions, the following provisions have been included:
(1)  
An output is provided from the main connector to the electrical power annunciator panel, figure 5, for sensing the operation of differential protection, overvoltage, undervoltage, overexcitation and underexcitation voltages.

(2)  
External monitoring of the stability protection circuit (where applicable) is provided for at a standard connector for a quick analysis of a faulted system. A diode is used to block external pickup which may cause false trip of the generator (field) control relay. An external monitoring amplifier of a vacuum tube voltmeter (VTVM) may be used to monitor the output of the sensing circuit.

D.  The bus-tie breaker (BTB) connect the generator load bus to the three phase tie bus. This permits operation of two generators by means of automatic switching or permits feeding the generator load bus from the APU or ground power. The BTB can be closed or tripped manually from the forward overhead panel, or can be tripped automatically by a fault signal in the generator control unit.
H20942
500  Generator Control Unit Functional Block Diagram 
Nov 15/69  Figure 4  24-21-0 
Page 10A/10B 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

E.  The voltage regulator section is in the generator control unit. It regulates the average of the three-phase voltages as determined at the generator feeder ends and limits the height of phase voltages under unbalanced load or fault conditions. All dc or excitation power supplied to the generator exciter is controlled by the voltage regulator for all load conditions. This dc power is furnished by a T-R unit within the generator control unit. The voltage regulator will limit the input power to the generator to 160 horsepower by means of power limiting current transformers on the generator output feeder lines. The scheme used in the voltage regulator to limit generator power output is to sense system voltage by means of the boost current transformers and line current by means of the power limiting current transformers. These two signals are then combined to obtain a signal which is approximately proportional to the real load on the system. The output of the power control circuit is connected into the voltage regulator circuit in such a manner so that, when system real load approaches a value sufficient to cause excessive generator input torque, the signal from the power control circuit takes over and reduces the system voltage. In this manner, the system real load is limited so that the critical torque is not exceeded.
F.  Protective functions of the generator control unit against abnormal system conditions are obtained by incorporating various sensing devices which supply a trip signal to the generator field (control) relay.
(1)  
Overvoltage protection is obtained by supplying a signal to the generator field relay when the average three-phase voltage exceeds 127 to 133 volts line-to-neutral.

(2)  
The undervoltage device applies a trip signal, also to the generator field relay within 5 to 9 seconds after the voltage drops and remains below 97 to 103 volts line-to-neutral on all three phases of the generator.

(3)  
The differential current sensing device and the connecting current transformers protect the generator, generator main bus and interconnecting feeders. Settings are high enough to prevent nuisance tripping and adjusted to trip the GFR in any event at 20 amperes differential current and above.

(4)  
The overcurrent protection locks out under frequency protection and trips the generator field relay through a time delay which holds long enough for the generator circuit breaker to open. The rating of the generator circuit breaker and the selection of the overcurrent current transformers is dependent on the generator rating, and are different for 30 and 40 kva machines. Overcurrent will lock out underfrequency protection.

500 
May 15/69  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  24-21-0 Page 11 

H34525 H20931
Electrical Power Annunciator Panel  5B6 
24-21-0 Page 12  Figure 5 (Sheet 1) BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Sep 20/82 

500  Electrical Power Annunciator Panel 
　
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