Overvoltage (59); Undervoltage (27); Over And Underfrequency (81); Voltage Transformer Fuse Failure (Vtff) - GE DGP Series Instruction Manual

1.3 PROTECTION FEATURES 1 PRODUCT DESCRIPTION

1.3.10 OVERVOLTAGE (59)

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This function consists of a positive-sequence overvoltage with an user selectable inverse or definite time char-
acteristic. See Figure 1–3: SIMPLE LOGIC DIAGRAM – 87G, 32, 27, 59, AND AE on page 1–12 for the logic
diagram and Figure 2–15: 59 TIME-VOLTAGE CHARACTERISTICS on page 2–35 for the inverse time-voltage
characteristics. A linear reset characteristic is incorporated for this function. The overvoltage function can be
considered as a backup to the Volts/Hz function. Some possible causes of this condition are a system distur-
bance or regulator failure.

1.3.11 UNDERVOLTAGE (27)

This function consists of a positive-sequence undervoltage with an user selectable inverse or definite time
characteristic. See Figure 1–3: SIMPLE LOGIC DIAGRAM – 87G, 32, 27, 59, AND AE on page 1–12 for the
logic diagram and Figure 2–17: 27 TIME-VOLTAGE CHARACTERISTICS on page 2–40 for the inverse time-
voltage characteristics. A linear reset characteristic is incorporated for this function.

1.3.12 OVER AND UNDERFREQUENCY (81)

This function provides over and underfrequency protection, each with an adjustable time delay. Two or four
over and underfrequency steps are provided depending on the model. All frequency functions are supervised
by an adjustable positive-sequence voltage level. This undervoltage cut-off level and/or digital input DI1 can be
used to block the frequency functions during start-up. Frequency disturbance can occur due to a system fault
or islanding of the unit or an unconnected unit can operate at abnormal frequency due to malfunction of speed
control. Figure 1–6: SIMPLE LOGIC DIAGRAM – 81-O AND 81-U on page 1–15 shows the logic diagram for
this function.

1.3.13 VOLTAGE TRANSFORMER FUSE FAILURE (VTFF)

Functions 40 and 51V may operate for a full or partial loss of AC potential caused by one or more blown fuses.
The DGP makes provisions to block tripping by these functions when a fuse failure is detected; all other protec-
tion functions are allowed to trip. Figure 1–7: SIMPLE LOGIC DIAGRAM – VT FUSE FAILURE on page 1–16
shows the logic diagram for the VTFF function.
If AC potential is lost on one or more phases, the negative-sequence voltage (V2) rises and/or the positive-
sequence voltage (V1) drops. Either V2 > 15V or V1 < 50V provides a basic indication of the VTFF condition.
This signal is supervised by a Disturbance Detector (DD) and generator positive-sequence current (I1) detec-
tor (see three-input AND gate on the logic diagram). Supervision by the DD and I1 signals provide security
against false operation during fault and generator out of service conditions respectively. Security is enhanced
by use of the A/0 and B/0 timers shown in the logic diagram.
Signal DD is derived from a combination of sequence current levels, change in levels, and pickup flags of vari-
ous protection functions as shown in the logic diagram.
The VTFF logic allows integration of an external VTFF contact. Either of the two fuse-failure signals or both
signals can be configured to block tripping of functions 40 and 51V.
Detection of VTFF energizes the 74FF (Fuse Failure alarm) relay, de-energizes the 74CR (critical alarm) relay,
and turns the status LED red, even though all protection functions except 40 and 51V are unaffected.
1- DGP Digital Generator Protection System GE Power Management
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