Operation Principle - ABB RELION REX640 Technical Manual

Section 4
Protection functions
4.2.5.5
482
troubleshooting, check the contents of this chapter and also the preprocessing blocks
in this document. The configuration can be written to the protection relay once the
mismatch is corrected.

Operation principle

The function can be enabled and disabled with the Operation setting. The
corresponding parameter values are "on" and "off".
The operation of MREFPTOC can be described using a module diagram. All the
modules in the diagram are explained in the next sections.
Level
Io
IRES
detector 1
Level
detector 2
BLOCK
GUID-EDC7764D-F8E2-4E1D-8F95-AA81EA83E4B7 V1 EN
Figure 264: Functional module diagram
Level detector 1
The measured rotor earth-fault current (DFT value) is compared to the Operate start
value setting. If the measured value exceeds that of the Operate start value setting,
Level detector 1 sends a signal to start the Timer 1 module.
Level detector 2
The measured rotor earth-fault current (DFT value) is compared to the set Alarm start
value. If the measured value exceeds that of the Alarm start value setting, Level
detector 2 sends a signal to start the Timer 2 module.
For MREFPTOC, the earth-fault current is the current that flows due
to the voltage injected by the injection device in the rotor circuit when
an earth fault arises.
A considerable amount of harmonics, mainly 3rd and 6th, can occur in
the excitation current under normal no-fault conditions, especially
with the thyristor excitation and rotating diode rectifier systems.
MREFPTOC uses DFT value calculation to filter DC and harmonic
components which could otherwise give out false alarms or trips.
Timer 1
OPERATE
t
START
Timer 2
t
ALARM
1MRS759142 C
REX640
Technical Manual