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.
4.3.6.5 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
s
)
and the 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
in
) and lead
resistance (R
m
).
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.
Protection functions
1MRS757644 H
598 620 series
Technical Manual
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