262
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LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
GROUPED PROTECTION ELEMENTS
CHAPTER 7: PROTECTION
Figure 222: Negative-sequence directional overcurrent scheme logic
Voltage elements
The voltage protection elements can be used for a variety of applications, such as
undervoltage protection, permissive functions, and source transfer schemes.
For voltage sensitive loads, such as induction motors, a drop in voltage increases the
drawn current which may cause dangerous overheating in the motor. The undervoltage
protection feature can be used to either cause a trip or generate an alarm when the
voltage drops below a specified voltage setting for a specified time delay.
For permissive functions, the undervoltage feature may be used to block the functioning of
external devices by operating an output relay when the voltage falls below the specified
voltage setting. The undervoltage feature may also be used to block the functioning of
other elements through the block feature of those elements.
In source transfer schemes, a transfer signal may be generated to transfer a load from its
normal source to a standby or emergency power source in the event of an undervoltage.
The undervoltage elements can be programmed to have a definite time delay
characteristic. The definite time curve operates when the voltage drops below the pickup
level for a specified period of time. The time delay is adjustable from 0 to 600.00 seconds in
steps of 0.01. The undervoltage elements can also be programmed to have an inverse time
delay characteristic.
The undervoltage delay setting defines the family of curves shown below.
Eq. 36
In this equation,
T
represents the operating time,
D
represents the undervoltage delay
setting (
D
= 0.00 operates instantaneously),
V
represents the secondary voltage applied to
the relay, and
V
pickup
represents the pickup level.
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