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1MAC050144-MB C
Section 4
Protection functions
615 series ANSI
343
Technical Manual
almost always when the transformer is connected to the network. Typically, the inrush
current contains a large amount of second harmonics.
Blocking the operation of the 87T restrained differential at a magnetizing inrush current is
based on the ratio of the amplitudes of the second harmonic digitally filtered from the
differential current and the fundamental frequency (Id2f /Id1f).
The blocking also prevents unwanted operation at the recovery and sympathetic
magnetizing inrushes. At the recovery inrush, the magnetizing current of the transformer
to be protected increases momentarily when the voltage returns to normal after the
clearance of a fault outside the protected area. The sympathetic inrush is caused by the
energization of another transformer running in parallel with the protected transformer
already connected to the network
The ratio of the second harmonic to a fundamental component can vary considerably
between the phases. Especially when the delta compensation is done for a Ynd1 connected
transformer and the two phases of the inrush currents are otherwise equal but opposite in
phase angle, the subtraction of the phases in a delta compensation results in a very small
second harmonic component.
Some measures have to be taken in order to avoid the false tripping of a phase having too
low a ratio of the second harmonic to the fundamental component. One way could be to
always block all the phases when the second harmonic blocking conditions are fulfilled in
at least one phase. The other way is to calculate the weighted ratios of the second harmonic
to the fundamental component for each phase using the original ratios of the phases. The
latter option is used here. The second harmonic ratios I_2H_RAT_x are given in
monitored data.
The ratio to be used for second harmonic blocking is therefore calculated as a weighted
average on the basis of the ratios calculated from the differential currents of the three
phases. The ratio of the concerned phase is of most weight compared to the ratios of the
other two phases. In this IED, if the weighting factors are four, one and one, four is the
factor of the phase concerned. The operation of the restrained differential on the concerned
phase is blocked if the weighted ratio of that phase is above the set blocking limit Pickup
value 2.H and if blocking is enabled through the Restraint mode parameter.
Using separate blocking for the individual phases and weighted averages calculated for the
separate phases provides a blocking scheme that is secure at the connection inrush
currents.
If the peak value of the differential current is very high, that is Ir> 12 p.u., the limit for the
second harmonic blocking is desensitized (in the phase in question) by increasing it
proportionally to the peak value of the differential current.
The connection of the power transformer against a fault inside the protected area does not
delay the operation of the tripping, because in such a situation the blocking based on the
second harmonic of the differential current is prevented by a separate algorithm based on
a different waveform and a different rate of change of the normal inrush current and the
inrush current containing the fault current. The algorithm does not eliminate the blocking
at inrush currents, unless there is a fault in the protected area.
Normally, there are low current periods in the differential current during inrush. Also the
rate of change of the differential current is very low during these periods. If these features
Summary of Contents for 615 Series ANSI
Page 1: ...Relion Protection and Control 615 series ANSI Technical Manual ...
Page 2: ......
Page 29: ...1MAC050144 MB C Section 615 series ANSI 22 Technical Manual ...
Page 389: ...Section 4 1MAC050144 MB C Protection functions 382 615 series ANSI Technical Manual ...
Page 485: ...Section 6 1MAC050144 MB C Supervision functions 478 615 series ANSI Technical Manual ...
Page 635: ...Section 10 1MAC050144 MB C Other functions 628 615 series ANSI Technical Manual ...
Page 711: ...Section 16 1MAC050144 MB C Glossary 704 615 series ANSI Technical Manual ...
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