Cvt Filter - ABB REL650 series Technical Manual

Section 6
Impedance protection
6.1.6.7
6.1.6.8
122
The apparent impedance is considered as an impedance loop with resistance R and
reactance X.
The formula given in equation
When load is considered in the case of single phase-to-ground fault, conventional distance
protection might overreach at exporting end and underreach at importing end. The IED
has an adaptive load compensation which increases the security in such applications.
The directional evaluations are performed simultaneously in both forward and reverse
directions, and in all six fault loops. Positive sequence voltage and a phase locked positive
sequence memory voltage are used as a reference. This ensures unlimited directional
sensitivity for faults close to the IED point.

CVT filter

In HHV networks, use of CVT is very common due to their reduced cost and size. But due
to the capacitance effect the voltage will not change instantaneously and this causes the
CVT transients into post fault voltage signals. Many times, this causes the relays to over
reach. ZQMPDIS (21) handles the CVT transients internally and if setting CVTFltr is
Enabled then input voltage profile is corrected accordingly.
CVT filter is designed to reduce the CVT transients. Use CvtFltr setting to enable or
disable the CVT filter. The filter detects the fault and switches the filter coefficient to give
correct voltage values to the measurement loops. When CvtFltr is Enabled all the loops
and all the zones will have the corrected voltage value. In applications where the line has
a high SIR and the voltage signals are obtained by CVT, it is highly recommended to use
the CVT Filter.
Simplified logic diagrams
Distance protection zones
The design of the distance protection zones are presented for all measuring loops: phase-
to-ground as well as phase-to-phase.
Phase-to-ground related signals are designated by AG, BG and CG. The phase-to-phase
signals are designated by AB, BC and CA.
Fulfillment of two different measuring conditions is necessary to obtain the one logical
signal for each separate measuring loop:
• Zone measuring condition, which follows the operating equations described above.
• Group functional input signal (PHSEL), as presented in figure 49.
24 is only valid for radial feeder application without load.
1MRK 506 335-UUS A
Technical manual