7SR10 Description of Operation
©2018 Siemens Protection Devices Limited Chapter 1 Page 64 of 77
≥
1
81THD
81THD
81THD Setting
c
81THD
Enabled
Disabled
>
c
81THD Delay
>
c
>
c
IL1
IL2
IL3
81THD
Measurement
L1 81HBL2
81THD- Inrush
Action
Off
Inhibit
&
L2 81HBL2
L3 81HBL2
&
&
≥
1
General Pickup
&
Figure 5-3 Logic Diagram: Total Harmonic Distortion Supervision Element (81THD)
5.4 VT Supervision (60VTS)
1 or 2 Phase Failure Detection
Normally the presence of negative phase sequence (NPS) or zero phase sequence (ZPS) voltage in a power
system is accompanied by NPS or ZPS current. The presence of either of these sequence voltages without the
equivalent level of the appropriate sequence current is used to indicate a failure of one or two VT phases.
The
60VTS Component
setting selects the method used for the detection of loss of 1 or 2 VT phases i.e.
ZPS
or
NPS
components. The sequence component voltage is derived from the line voltages; suitable VT connections
must be available. The relay utilises fundamental voltage measurement values for this function.
The element has user settings
60VTS V
and
60VTS I.
A VT is considered to have failed where the voltage
exceeds
60VTS V
while the current is below
60VTS I
for a time greater than
60VTS Delay
.
3 Phase Failure Detection
Under normal load conditions rated PPS voltage would be expected along with a PPS load current within the
circuit rating. Where PPS load current is detected without corresponding PPS voltage this could indicate a three
phase VT failure. To ensure these conditions are not caused by a 3 phase fault the PPS current must also be
below the fault level.
The element has a
60VTS VPPS
setting, an
60VTS IPPS Load
setting and a setting for
60VTS IPPS Fault
. A VT
is considered to have failed where positive sequence voltage is below
60VTS VPPS
while positive sequence
current is above
IPPS Load
and below
IPPS Fault
level for more than
60VTS Delay
.
External MCB
A binary input can be set as
Ext_Trig 60VTS
to allow the
60VTS Delay
element to be started from an external
MCB operating.
Once a VT failure condition has occurred the output is latched on and is reset by any of the following:-
Voltage is restored to a healthy state i.e. above
VPPS
setting while NPS voltage is below
VNPS
setting.
Ext Reset 60VTS
A binary or virtual input, or function key and a VT failure condition no longer
exists.
Inhibit 60VTS
A binary or virtual input.
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