Voltage Elements; Undervoltage Curves; Phase Undervoltage Protection - GE 845 Instruction Manual

CHAPTER 4: SETPOINTS

Voltage Elements

Undervoltage Curves

NOTE:
Phase Undervoltage
Protection
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
Figure 4-80: Voltage Elements Display Hierarchy
The undervoltage elements can be programmed to have an inverse time delay
characteristic. The undervoltage delay setpoint defines a family of curves as shown below.
The operating time is given by:
T = D/(1 - V/V )
pkp
Where:
T = Operating Time
D = Undervoltage Pickup Time Delay setpoint (for D = 0.00 operates instantaneously)
V = Voltage as a fraction of the nominal VT Secondary Voltage
V = Undervoltage Pickup Level
pkp
The element resets instantaneously if the applied voltage exceeds the dropout voltage. The
delay setting selects the minimum operating time of the phase undervoltage.
At 0% of Pickup, the operating time equals the Undervoltage Pickup Time Delay setpoint.
Figure 4-81: Inverse Time Undervoltage Curves
The 845 relay is equipped with the Phase Undervoltage (UV) element. The Phase
Undervoltage element may be used to protect voltage sensitive loads and system
components against sustained undervoltage conditions. This element may be used for
permissive functions, initiation of the source transfer schemes, and similar functions.
The Phase Undervoltage element may be set as an instantaneous element with no time
delay or as a time delayed element which can be programmed with definite time or
inverse time. The Phase Undervoltage element has programmable minimum operating
threshold to prevent some undesired operation when voltage is not available. The input
voltages are the three phase to phase voltages from delta connected VTs (PTs) or three
phase to ground voltages from wye connected VTs (PTs).
The settings of this function are applied to each of the three voltage inputs to produce
Pickup and Trip flags per voltage input. The UV Pickup flag is asserted, when the measured
voltage on any of the three voltage inputs is below the PKP value. The UV Trip flag is
asserted if the element stays picked up for the time defined by Pickup time delay or for the
time defined by the selected inverse curve, and number of voltages required for operation
PROTECTION
4–203