4.4.4.10
Technical data
Table 768: PREVPTOC Technical data
Characteristic
Value
Operation accuracy
Depending on the frequency of the measured current: f
n
±2 Hz
±1.5% of the set value or ±0.002 × I
n
Start time
1, 2
I
Fault
= 2.0 × set
Start value
Minimum
Typical
Maximum
23 ms
25 ms
28 ms
Reset time
Typically 40 ms
Reset ratio
Typically 0.96
Retardation time
<35 ms
Operate time accuracy in definite time mode
±1.0% of the set value or ±20 ms
Suppression of harmonics
DFT: -50 dB at f = n × f
n
, where n = 2, 3, 4, 5,…
4.4.5
Negative-sequence overcurrent protection for machines
MNSPTOC (ANSI 46M)
4.4.5.1
Identification
Function description
IEC 61850
identification
IEC 60617
identification
ANSI/IEEE C37.2
device number
Negative-sequence overcurrent pro-
tection for machines
MNSPTOC
I2>M
46M
4.4.5.2
Function block
Figure 476: Function block
4.4.5.3
Functionality
The negative-sequence overcurrent protection for machines function MNSPTOC
protects electric motors from phase unbalance. A small voltage unbalance can
produce a large negative-sequence current flow in the motor. For example, a 5
percent voltage unbalance produces a stator negative-sequence current of 30
percent of the full load current, which can severely heat the motor. MNSPTOC
detects the large negative-sequence current and disconnects the motor.
The function contains a blocking functionality. It is possible to block the function
outputs, timers or the function itself.
1
Negative-sequence current before = 0.0, f
n
= 50 Hz, results based on statistical distribution of
1000 measurements.
2
Includes the delay of the signal output contact.
1MRS759142 F
Protection functions
REX640
Technical Manual
841