ABB Relion REC615, Техническое руководство

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DFT
The DFT measurement principle is selected with the
Measurement mode setting
using the value "DFT". In the DFT mode, the fundamental frequency component
of the measured signal is numerically calculated from the samples. In some
applications, for example, it can be difficult to accomplish sufficiently sensitive
settings and accurate operation of the low stage, which may be due to a
considerable amount of harmonics on the primary side currents. In such a case,
the operation can be based solely on the fundamental frequency component of the
current. In addition, the DFT mode has slightly higher CT requirements than the
peak-to-peak mode, if used with high and instantaneous stages.
Peak-to-peak
The peak-to-peak measurement principle is selected with the
Measurement mode
setting using the value "Peak-to-Peak". It is the fastest measurement mode, in which
the measurement quantity is made by calculating the average from the positive
and negative peak values. The DC component is not included. The retardation time
is short. The damping of the harmonics is quite low and practically determined
by the characteristics of the anti-aliasing filter of the protection relay inputs.
Consequently, this mode is usually used in conjunction with high and instantaneous
stages, where the suppression of harmonics is not so important. In addition,
the peak-to-peak mode allows considerable CT saturation without impairing the
performance of the operation.
Peak-to-peak with peak backup
The peak-to-peak with peak backup measurement principle is selected with the
Measurement mode setting using the value "P-to-P+backup". It is similar to the
peak-to-peak mode, with the exception that it has been enhanced with the peak
backup. In the peak-to-peak with peak backup mode, the function starts with two
conditions: the peak-to-peak value is above the set start current or the peak value
is above two times the set
Start value. The peak backup is enabled only when
the function is used in the DT mode in high and instantaneous stages for faster
operation.
11.6 Calculated measurements
Calculated residual current and voltage
The residual current is calculated from the phase currents according to equation:
Io I I I
A B C
= − + +
( )
(Equation 89)
The residual voltage is calculated from the phase-to-earth voltages when the VT
connection is selected as “Wye” with the equation:
Uo U U U
A B C
= + +
( ) / 3
(Equation 90)
General function block features 1MRS758755 C
828 REC615 & RER615
Technical Manual