Switch Onto Fault Feature; Setting Example; Introduction - ABB RELION 650 Series Applications Manual

Section 6
Differential protection
OpenCT : Open CT detected
•
• OpenCTAlarm : Alarm issued after the setting delay
OpenCTIN : Open CT in CT group inputs (1 for input 1 and 2 for input 2)
•
OpenCTPH : Open CT with phase information (1 for phase L1, 2 for phase L2, 3 for phase L3)
•
6.1.2.9

Switch onto fault feature

The Transformer differential function in the IED has a built-in, advanced switch onto fault
feature. This feature can be enabled or disabled by the setting parameter
SOTFMode = On this feature is enabled. It shall be noted that when this feature is enabled it is
not possible to test the 2
superimposed second harmonic. In that case the switch on to fault feature will operate and
the differential protection will trip. However for a real inrush case the differential protection
function will properly restrain from operation.
For more information about the operating principles of the switch onto fault feature please
read the Technical Manual.
6.1.3

Setting example

6.1.3.1

Introduction

Differential protection for power transformers has been used for decades. In order to correctly
apply transformer differential protection proper compensation is needed for:
• power transformer phase shift (vector group compensation)
• CT secondary currents magnitude difference on different sides of the protected
transformer (ratio compensation)
• zero sequence current elimination (zero sequence current reduction) shall be done. In the
past this was performed with help of interposing CTs or special connection of main CTs
(delta connected CTs). With numerical technology all these compensations are done in IED
software.
The Differential transformer protection is capable to provide differential protection for all
standard three-phase power transformers without any interposing CTs. It has been designed
with assumption that all main CTs will be star connected. For such applications it is then only
necessary to enter directly CT rated data and power transformer data as they are given on the
power transformer nameplate and differential protection will automatically balance itself.
However the IED can also be used in applications where some of the main CTs are connected in
delta. In such cases the ratio for the main CT connected in delta shall be intentionally set for √
(3)=1.732 times smaller than actual ratio of individual phase CTs (for example, instead of 800/5
set 462/5) In case the ratio is 800/2.88A, often designed for such typical delta connections,
set the ratio as 800/5 in the IED. At the same time the power transformer vector group shall
be set as Yy0 because the IED shall not internally provide any phase angle shift compensation.
The necessary phase angle shift compensation will be provided externally by delta connected
main CT. All other settings should have the same values irrespective of main CT connections.
70
nd
harmonic blocking feature by simply injecting one current with
These are internal compensations within the differential function. The
protected power transformer data are always entered as they are given on the
nameplate. Differential function will by itself correlate nameplate data and
properly select the reference windings.
1MRK 504 158-UEN A
M15266-348 v5
SOTFMode . When
SEMOD167745-1 v1
SEMOD167720-5 v6
Application manual