Reset Curves
The fault reset time for an ACR is normally a definite time setting configurable between 0 and 10000 ms. The default value is 50 ms.
The Reset Curves feature provides an alternative way of determining the time taken for a fault to reset. It allows an ACR to emulate an induction disc relay and thus coordinate with an upstream device of this type.
The time it takes for a fault to reset following a protection pickup is determined by the Reset Curve and Time Multiplier for the element of the active trip as well as the magnitude of current still flowing in that element.
Reset Curves can be made available through WSOS in Feature Selection or via the operator interface.
When Reset Curves are made available, the Fault Reset Time setting is removed from the WSOS Global Settings page.
An additional setting is added to allow the reset curve to start at Pickup or Threshold.
Figure 132
Global Settings with Reset Curve
Selected
A less obvious change is the Reset Threshold Multiplier setting range for each element that is extended from 90% ... 100% to 10% ... 100%.
Phase Reset Curve settings are also added to the trip settings for each element for Trips 1...4, Single Shot and Work Tag.
By default all of these settings are 50 ms Definite Time which provides the same behavior as when Reset Curves are not available.
Definite Time can be configured for between 0 s and 100 s with a 10 ms resolution. Instantaneous Only is also an option, as well as an inverse time curve.
Any one of six standard inverse time curves can be selected:
IEC255 Standard Inverse
IEC255 Very Inverse
IEC255 Extremely Inverse
IEEE Standard Moderately Inverse
IEEE Standard Very Inverse
IEEE Standard Extremely Inverse
Tables listing the reset times and formula for calculating these times can be found in Appendix G – Reset Curve Timing Tables.
The figure below shows the Trip 1 settings with Reset Curve settings applied.
Figure 133
Trip 1 Reset Curve settings
When Reset Curves are used, the Curve Viewer in WSOS displays both the Pickup and Reset curves on a single graph.
The curves displayed will change to reflect the applied global settings for:
-
Setting Current
-
Pickup Multiplier
-
Reset Threshold Multiplier
-
Reset Curve Start at
Reset Curve Start at
There are two options concerning where the reset curve must start. They are:
-
Pickup – the curve is plotted from the setting current x pickup multiplier.
-
Threshold – the curve is plotted from the pickup current x reset threshold multiplier.
If the Reset Threshold Multiplier setting = 100%, the Reset Curve Start at
setting will have no effect because the pickup and reset currents
are the same that is 200 A when the settings below are applied. 
Figure 134
Pickup and Reset Curves with 100%
Reset Threshold Multiplier
If the Reset Threshold Multiplier setting is changed to 80%, the pickup value will remain at 200 A but the reset threshold will change to 160 A, that is 80% of 200 A.
If the reset curve is set to start at threshold, the shape of the reset curve will not change but its starting point will move from 200 A to 160 A as shown below.
Figure 135
80% Reset Threshold Multiplier
and Reset Curve starting at Threshold
Changing the Reset Curve Start at setting to Pickup will move the reset curve starting point back to 200 A, but the effective curve will still start at 160 A due to the Reset Threshold Multiplier setting of 80%. This changes the shape of the reset curve because it is truncated at 160 A.
So for currents less than the reset threshold (160 A in this case) reset time will be the same when the reset curve starts at pickup.
Figure 136
Pickup and Reset Curves with Reset
Curve Start at Pickup
Reset Curve Example
This example describes the sequence of events that occur when a permanent threephase 400 A per phase fault is detected downstream from an ACR with the protection settings shown in the figure below.
Figure 137
Protection Settings for an ACR
with Reset Curves Available
400 A is 2 x the Phase Setting Current so according to
the Trip 1 Phase Protection curve (IEC 255 SI) and Time Multiplier
(0.5) this will cause the ACR to trip in
5.015 s.
Refer to Appendix D for IEC 255 Inverse Time Tables.
When the ACR trips, the current will go to zero and the fault reset timing will start. According to the Trip 1 Time Multiplier (0.5) and Reset Curve (IEC 255 SI) the reset time must be 6.75 s. Refer to Appendix G – Reset Curve Timing Tables.
The Reclose Time setting for Trip 1 is 4.5 s. This is shorter that the fault reset time (6.75 s), so the fault will not have fully reset when the ACR automatically recloses. The fault will have two thirds reset (4.5/6.75) which is analogous to an induction disc for a protection relay having travelled two thirds of the way back to its rest position.
When the ACR closes back onto the
400 A fault, it will record a Protection Element Timing event in the Event Log. Instead of a trip time of 5.015 s as occurred
for Trip 1, the trip time for Trip 2 will be two thirds of
that 3.34 s (5.015 x 0.667).