Parameter: VTS Inhibit
•
Default setting: No
This setting defines the effect of a voltage transformer failure on the operation of each 27 element. If a
VT failure is detected by the separate VTS element, the element will be automatically blocked if this
setting is set to Yes. If this setting is set to No, the 27-n element will be unaffected by the VTS element
operation and the 27-n element may operate on the erroneous voltage caused by the VT fuse failure
which has been correctly identified by VTS. It is generally recommended that using the VTS Inhibit func-
tion is beneficial.
Settings Example
2-Stage Undervoltage Protection
This example shows settings that could be applied for a 2-stage undervoltage protection application.
Settings are applied to detect a condition of 10 % reduction in supply voltage compared to nominal as a
standing level which may indicate a voltage control failure but also detects a 30 % reduction in voltage
between any phases more quickly to allow remedial actions such as a supply changeover. The actions should
not be initiated for a loss of supply which could result from a grid supply trip which is assumed to be handled
separately. This example considers device settings for the 27 elements but does not cover I/O mappings and
tripping logic.
•
First stage
To detect a -10 % deviation in the voltage compared to the expected 110 V phase to phase secondary
nominal voltage allowing for a voltage controller which would be expected to correct this deviation in
less than 2 seconds.
Threshold = 110 ⋅ 0.9 = 99 V
Time delay must be greater than 2 seconds. Choose 3 s.
Choose a suitable element: 27-1
•
Second stage
To detect a -30 % deviation in the voltage compared to the expected 110 V phase to phase secondary
nominal voltage allowing only for transient conditions and fault clearance and meeting the customers’
expectations for supply changeover which will allow 1000 ms.
Threshold = 110 ⋅ 0.7 = 77 V
Time delay specified as 1000 ms.
Choose a suitable element: 27-2
•
Immunity to Loss of Supply
Set UV guard to a low level that would not be expected unless the system is dead. Choose 5 V.
•
Immunity to VT Fail
Both of these elements will be affected by VT failure if these settings are applied but a 3 phase VT fail
which would be required to affect the first stage may be less likely. In many cases it is preferable to
inhibit the voltage protection.
•
Any/All phases
The voltage control will operate on all 3 phases and the failure will result in voltage reduction on all 3
phases. The first stage should be set to operate on All phases. The second stage must operate for under-
voltage on Any phase.
Element
Setting Values
27 Common Input Selection Ph-Ph
27 Common UV Guard Setting 5 V
27-1 Element Enabled
27-1 Setting 99 V
27-1 Hysteresis 3 %
27-1 Delay 3 s
Protection and Automation Functions
5.2 27 Undervoltage Protection – 3-Phase
Reyrolle 7SR5, Overcurrent Protection Device, Device Manual 153
C53000-G7040-C014-1, Edition 11.2019
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