ABB RELION 620 Series Technical Manual page 693

1MRS757644 F
620 series
Technical Manual
GUID-722239A2-AF2E-4E0A-A0B1-95AE99F24CF2 V1 EN
Figure 348: Functional module diagram
LVRT curve monitoring
LVRT curve monitoring starts with detection of undervoltage. Undervoltage
detection depends on Voltage selection setting. All selectable options are based on
fundamental frequency components.
Function uses phase-to-earth voltages when Voltage selection is set to "Highest Ph-to-
E" or "Lowest Ph-to-E" and phase-to-phase voltages when Voltage selection is set to
"Highest Ph-to-Ph" or "Lowest Ph-to-Ph".
When the Voltage selection setting is set to "Highest Ph-to-E", "Lowest Ph-to-E",
"Highest Ph-to-Ph" or "Lowest Ph-to-Ph", the measured three-phase voltages are
compared phase-wise to the set Voltage start value. If the measured value is lower
than the set Voltage start value setting in number of phases equal to that set Num of
start phases, the START output is activated.
The setting options available for Num of start phases are "Exactly 1 of 3", "Exactly 2
of 3", and "Exactly 3 of 3", which are different from conventional setting options
available in other functions. For example, Num of start phases is set to "Exactly 2 of
3", any two voltages should drop below Voltage start value within one cycle network
for the START output to activate. Even if more than two voltages drop below Voltage
start value, START output is not activated.
When the Voltage selection setting is "Positive Seq", the positive-sequence
component is compared with the set Voltage start value. If it is lower than the set
Voltage start value, the START output is activated.
Once START is activated, the function monitors the behavior of the voltage defined by
Voltage selection setting with the defined LVRT curve. When defined voltage enters
the operating area, the OPERATE output is activated instantaneously. The pulse length
of OPERATE is fixed to 100 ms. START also deactivates along with OPERATE.
If a drop-off situation occurs, that is, voltage restores above Voltage start value, before
OPERATE is activated, the function does not reset until maximum recovery time
under consideration has elapsed, that is, START output remains active.
LVRT curve is defined using time-voltage settings coordinates. The settings available
are Recovery time 1...Recovery time 10 and Voltage level 1...Voltage level 10. The
number of coordinates required to define a LVRT curve is set by Active coordinate
settings.
Section 4
Protection functions
687