ABB RELION REX640, Техническое руководство, Страница: 807 / 1960

4.3.8.5 Operation principle
The function can be enabled and disabled with the
Operation setting. The
corresponding parameter values are "On" and "Off".
The operation of MHZPDIF can be described using a module diagram. All the
modules in the diagram are explained in the next sections.
OPERATE
START
BLOCK
t
Timer
Level
detector
I3P
I_A
I_B
I_C
Figure 459: Functional module diagram
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.
4.3.8.6 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
1MRS759142 F Protection functions
REX640
Technical Manual
807