Adaptive Sampling Frequency; Trip Circuit Monitor; Sequence Of Events - GE DGP Series Instruction Manual

1 PRODUCT DESCRIPTION 1.4 OTHER FEATURES
The components tested in the background are listed in Table 6–2: RUN-TIME BACKGROUND SELF-TESTS
on page 6–3. The testing of I/O hardware is done in the foreground so the processors know when a given com-
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ponent or port is in use and therefore not available for testing. The components tested in the foreground are
listed in Table 6–3: RUN-TIME FOREGROUND SELF-TESTS on page 6–3. Some foreground tests are per-
formed every sample period while others are performed less frequently. As with background self-tests, any
failed test is repeated and must fail three consecutive times to be considered a failure. Although not specifically
a self-test, trip circuit continuity monitoring is also performed as a foreground test. Refer to the TRIP CIRCUIT
MONITOR section below.
In addition to background self-tests, the operator may initiate a visual-response test of the MMI components.
Refer to Section 4.6.2 T1: MMI STATUS AND DISPLAY TESTING on page 4–9 for details.

1.4.5 ADAPTIVE SAMPLING FREQUENCY

The DGP system samples analog input waveforms at a rate of 12 samples per cycle. An adaptive sampling fre-
quency is used to maintain this rate over the power system frequencies of 30.5 to 79.5 Hz. As a result of this
feature, the measurement accuracy of the analog inputs and the sensitivities of the protection functions are
maintained over the range of power system frequencies. This feature provides improved protection for faults
during off-normal frequencies (such as start-up conditions). Figure 1–8: FREQUENCY-SENSITIVITY CHAR-
ACTERISTICS shows variations in sensitivity of protection functions at different power system frequencies.
The sampling frequency is based on 30.5 Hz for power system frequencies below 30.5 Hz and 79.5 Hz for the
frequencies above 79.5 Hz. In either case, if the AC voltage to the DGP drops below approximately 20 V, the
sampling frequency is automatically recalculated on the basis of the nominal system frequency (Setting 102:
SYSFREQ).
The sampling frequency, which is 12 times the measured system frequency, can be accessed as one of the
Present Values.

1.4.6 TRIP CIRCUIT MONITOR

The trip circuit monitor consists of DC voltage and current monitors (TVM and TCM respectively). Each of the
trip contacts shown with polarity marks in the elementary diagrams (see Section 1.5: ELEMENTARY DIA-
GRAMS) is monitored. TVM and TCM can be selectively disabled for each of the trip circuits.
Under normal conditions, DC voltage across each of the contacts is continuously monitored. If the DC voltage
becomes virtually zero, then the trip circuit has "failed open". The TVM is active only when the generator is on-
line, as indicated by the input DI1. This function is intended to replace the indicating light typically used for trip
circuit monitoring. It is universally rated for 48 through 250 V DC. A non-critical alarm is generated when the
TVM detects an abnormality.
When the DGP system issues a trip, DC current through each of the appropriate trip contacts is monitored. The
trip relay is sealed-in, as long as the current is flowing, to protect the contact. A minimum current of 150 mA is
required for the TCM to recognize the trip current. Status of the trip current flow following issuance of any trip is
logged in the sequence of events.

1.4.7 SEQUENCE OF EVENTS

This function time-tags and stores the last 100 events in memory. The resolution of the time-tagging is 1 milli-
second. The event list contains power system events, operator actions, and self-test alarms. The sequence of
events can be accessed, either locally or remotely, by a PC via one of the RS232 ports. A full description of this
function is contained in the Chapter 8: INTERFACE.
GE Power Management DGP Digital Generator Protection System 1-
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