ABB RELION 620 Series Technical Manual page 596

Section 4
Protection functions
4.3.6.5
590
Level detector
This module compares the three-phase differential currents to the set Operate value.
If any of the differential currents ID_A, ID_B or ID_C exceeds the set Operate value,
the Level detector module sends an enable signal to the Timer module to start the
definite timer (DT).
Timer
Once activated, the Timer activates the START output. The Timer characteristic is
according to DT. When the operation timer has reached the value set by Minimum
operate time, the OPERATE output is activated. If the fault disappears before the
module operates, the reset timer is activated. If the reset timer reaches the value set by
Reset delay time, the operation timer resets and the START output is deactivated.
The Timer calculates the start duration value START_DUR, which indicates the
percentage ratio of the start situation and the set operation time. The value is available
in the Monitored data view.
The activation of the BLOCK signal resets the Timer and deactivates the START and
OPERATE outputs.
Application
MHZPDIF provides the winding short circuit and earth-fault protection for motors.
The high-impedance or flux-balance principle has been used through many years for
differential protection due to the capability to manage through-faults with a heavy
current transformer (CT) saturation.
High-impedance principle
The high-impedance principle is stable for all types of faults outside the protection
zone. The stabilization is obtained by a stabilizing resistor in the differential circuit.
This method requires all the CTs to have a similar magnetizing characteristic, same
ratio and a relatively high knee point voltage. The CTs in each phase are connected in
parallel with a relay measuring branch. The measuring branch is a series connection
of the stabilizing resistor and the protection relay.
The stability of the protection is based on the use of the stabilizing resistor (R
fact that the impedance of the CT secondary quickly decreases as the CT saturates.
The magnetization reactance of a fully saturated CT drops to zero and the impedance
is formed only by the resistance of the winding (R
The CT saturation causes a differential current which can flow through the saturated
CT, because of the near-zero magnetizing reactance, or through the measuring branch.
The stabilizing resistor is selected so that the current in the measuring branch is below
the protection relay's operating current during out-of-zone faults. As a result, the
operation is stable during the saturation and can still be sensitive at the undistorted
parts of the current waveform.
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