(e)  
The AGCU also controls the AGCR, a relay in the AGCU which opens and closes the generator field circuit; and the APB, a latching-type relay which connects the APU generator to the sync bus. The AGCR and APB are closed when no fault conditions exist and they can be opened automatically when required.

 (f)  
The voltage regulation portion of the AGCU maintains the generator voltage at the point of regulation. The regulator senses the voltage at the point of regulation and compares this against a reference voltage. If a difference exists, it will adjust the generator exciter field current to maintain a constant voltage at the point of regulation.

 (g)  
Overvoltage (OV) protection is accomplished by sampling the three generator point of regulation (POR) phases. An OV condition exists when the highest of the three POR phase voltages is greater than the OV threshold. The 3-phase minimum actuating level for an OV protective trip is 130 |3 volts ac for POR frequency between 350 and 440 Hz. When OV is detected, a timer function is implemented based on the amount of OV. As the amount of OV increases, the time between the fault occurrence and the protective trip decreases. An OV condition shuts down the generator by tripping the AGCU, AGCR, and the APB.

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FUNCTION  SENSING  THRESHOLD  TIME DELAY  TRIPS  REMARKS
OVERVOLTAGE (OV)  HIGHEST PHASE POR VOLTAGE  >130 |3 VOLTS  INVERSE  AGCR, APB 
UNDERVOLTAGE (UV)  3-PHASE POR AVERAGE OF VOLTAGES  <104.5 |1.5 VOLTS  9 |1 SEC  AGCR, APB  LOCKED OUT BY US 
UNDERSPEED  APU UNDERSPEED  APU SPEED LESS  0.25 TO  APB  LOCKS OUT UV 
(US)  SWITCH  THAN 95% <380 Hz  0.60 
SECONDS 
DIFFERENTIAL CURRENT  GENERATOR DPCT AND LINE  20 |5 AMPS ABSOLUTE  0.04 SECONDS  AGCR, APB 
PROTECTION  DPCT  DIFFERENCE 
(DP)  BETWEEN GENERATOR CURRENT AND SUM 
OF LOAD AND 
SYNC BUS CURRENT 
POWER READY  NONE  OV, UV, US, OC  MSEC 150 |50  APB WILL CLOSE IF NO TRIPS ARE PRESENT 

Auxiliary Generator Control Unit (AGCU) Protective Functions
Figure 5

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(i)

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Undervoltage (UV) protection is accomplished by sampling the average of 3-phase POR voltages. The sensing circuitry may be combined with underexcitation protection sensing. The UV condition exists when 3-phase average POR voltage is less than
 104.5 |1.5 volts ac. When this condition exists, UV protective circuitry inhibits APB closure and trips the AGCR in 9 |1 seconds. 1) The UV function is inhibited while the underspeed
 protection is active or when the GCR is open.The underspeed (US) relay operates as a slave relay to the APU underspeed switch. When the APU drive output is abnormal, less than 95 percent, the underspeed switch will be closed and will provide a ground signal by which the US relay shall be energized. This signal will be common to both APU generator channels. The US relay, when energized, will, after a time delay of 0.25 to 0.60 seconds, energize the APB trip relay to trip the associated APB. A lockout shall be provided to lock-out the undervoltage protection upon initiation of the underspeed time delay.The differential (current) protection (DP) circuitry in the AGCUs monitors associated channels to protect against generator, generator feeder, APB, and APU generator bus faults. Differential protection circuitry compares generator and line currents at the generator current transformer and line DPCT. On any given phase, both generator and line currents should be identical. A low impedance to ground on one or more feeders between the generator and line CT indicates a differential current fault (the generator current is greater than the line current). Protection trip occurs if the maximum differential fault current is greater than 20 |5 amps. If such a condition persists for more than 0.04 seconds, protective circuitry trips the AGCR and APB.Power Ready 1) Power from the APU generator is available (ready) when the
　
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