Volts Per Hertz (24) - GE 845 Instruction Manual

CHAPTER 4: SETPOINTS

Volts per Hertz (24)

845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
The per-unit volts-per-Hertz (V/Hz) value is calculated using the maximum of the three-
phase voltage inputs or the auxiliary voltage channel Vx input, if the source is not
configured with phase voltages. To use the V/Hz element with the auxiliary voltage, set the
Signal Input to "Aux VT Bnk1-J2". If there is no voltage on the relay terminals in either case,
the per-unit V/Hz value is automatically set to "0". The per unit value is established based
on the voltage and nominal frequency power system settings as follows:
1. If the phase voltage inputs defined in the source menu are used for V/Hz operation,
then "1 pu" is the selected
Voltage 1 > Phase VT Secondary
System > Nominal Frequency
For example, if Phase VT Secondary and Nominal Frequency are set as 120 V and 60
Hz, respectively, these set values define the base unit as 1 x (V/Hz). The volts-per-hertz
ratio after division of these nominal settings is 120/60 = 2. If the Pickup setpoint from
the V/Hz element is set to "1.05 x (V/Hz)", will mean that in order for the element to pick
up, the actual volts-per-hertz ratio after division should be 2 *1.05 = 2.1. The ratio of
2.1 can be achieved if for example the measured voltage is 126V and frequency is 60
Hz, or the voltage is constant at 120 V and the frequency is 57.14 Hz, or any other
combination of these two values, which after V/Hz division equals 2.1. To check back
the pickup setting, we use the base (V/Hz) unit = 120/60 = 2, such that the pickup
setting value is 2.1/2 = 1.05 x (V/Hz).
2. When the auxiliary voltage Vx is used (regarding the condition for "None" phase
voltage setting mentioned above), then the 1 pu value is the
Voltage Sensing > Ph VT Bnk1-J2 > Voltage 1-J2 > Aux. VT Secondary
the
Setpoint > System > Power System > Nominal Frequency
3. If the V/Hz source is configured with both phase and auxiliary voltages, the maximum
phase among the three voltage channels at any given point in time is the input voltage
signal for element operation, so the per-unit value is calculated as described in item 1,
see previous items. If the measured voltage of all three phase voltages is "0", then the
per-unit value becomes automatically "0" regardless of the presence of an auxiliary
voltage.
The element has a linear reset characteristic. The reset time can be programmed to match
the cooling characteristics of the protected equipment. The element will fully reset from
the trip threshold in Reset Time seconds. The V/Hz element can be used as an
instantaneous element with no intentional time delay or as a Definite or Inverse timed
element. The characteristics of the inverse curves are shown as follows.
Path: Setpoints > Protection > Group 1(6) > Voltage > Volts per Hertz 1(2)
FUNCTION
Range: Disabled, Trip, Alarm, Latched Alarm, Configurable
Default: Disabled
This setting enables the Volts per Hertz functionality.
SIGNAL INPUT
Range: dependant on the order code
Default: Ph VT Bnk1-J2
This setting specifies the input voltage used to calculate the per-unit volts-per-Hertz (V/
Hz). If three phase voltages are used then set the AC Inputs to "Ph VT Bnk1-J2". To use the
V/Hz element with auxiliary voltage, set AC Inputs to "Aux VT Bnk1-J2".
Setpoint > System > Voltage Sensing > Ph VT Bnk1-J2 >
setting, divided by the
setting.
PROTECTION
Setpoint > System > Power
Setpoint > System >
setting divided by
setting.
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